I think your update changes the question significantly (probably too much for a SE question), but the answer is still essentially the same. In order for a miner to prevent a specific address from moving their BTC, they must not only decide not to mine the transaction but they must also convince at least 51% (most likely more) of the hashing power to not only not mine the transaction but also to ignore any block that does include that transaction.
First of all, I think that this is highly unlikely. Because:
Any combination of entities with >51% of the hashpower also has a very vested interest in Bitcoin. And deliberately allowing extortion would be against the interests of Bitcoin.
Any combination of entities with >51% of the hashpower has other potential ways of exploiting the network, which are arguably a lot more profitable.
The bribery incentive you list wouldn't really work. There's too much anonymity in the system. If you had an address that would give me back 110% of what I sent it, I would send it some money, launder the results into a new address, send it some more money from that untainted address, send it some more money, rinse and repeat.
Even if the "cartel" has 55% hashpower, there is a 1.8% chance that the minority miners could get six consecutive blocks mined. This may not sound like a lot, but if the extorted party just kept pushing trasactions out there it would only be a matter of a few hours before one of the transactions got six confirmations. So, the cartel would either have to be willing to break long strings of blocks (completely putting the network in jeopardy) or they would have an overwhelming supermajority of the hashpower (>90%).
So, in answer to your subquestions:
Is it illegal? Well Bitcoin doesn't have laws. If you mean, "would the protocol allow it?" Sure. A known weakness of the protocol is that if any entity controls a majority of the hashpower then there are some attacks they can make. Luckily there are some checks on this, notably the above comment: that anyone with a majority of the hashpower also has a lot invested into the health of the network. The deliberate limiting BTC Guild did on itself a few weeks ago shows this in action: the very entities that could obtain 51% hashpower are the ones most interested in making sure that no one obtains 51% hash power.
How would it be detected? In your modified scenario, it would be public knowledge. But even if someone tried to build a silent conspiracy, it would be pretty easy to detect if they went through with their threats: there would be orphaned blocks. And all of the orphaned blocks would share two things in common: they would all contain the extorted address and they would all be mined by minority miners. It would be pretty easy to figure out what is going on. Of course, you couldn't stop someone from secretly making threats, but detecting someone carrying out the threat would be easy.
How could it be resolved? Fundamentally the network is controlled by a majority of the hash power. If a supermajority of the miners decided to block an address, then there is no resolution: the majority rules. In a way, this is a feature. No one with the majority of the hash power would do this indiscriminately: it would jeopardize the network. But if the network determined that a particular address or addresses was a threat to the network, then the miners could band together and defend the network from that threat.
To expand on #3 with an example, say that a child pornographer publicly advertised some kind of service and accepted Bitcoin as payment. If, either out of altruism or media pressure, the majority of the hashpower decided to forbid payments to or from that address it would certainly be possible. (Of course, it would then become address "wack a mole", but there are some ways around that if the network could agree.)
TL;DR: The point being that if the miners collectively agree on something, yes, it is possible to effectively control the network. By design. That includes rolling back to a previous version (such as in the fork of March), preventing a theoretical exploit, or even executing a theoretical ban. But it's unlikely that a coalition would be willing to use that power for extortion. The miners have too much at stake in the health of the network.
I discovered something to add while researching something else. Satoshi made the commment that if transactions are deliberately being dropped that it could be addressed client side by coding the standard client to ignore blocks that they know are ignoring transactions. It was in a slightly different context in that thread, but the gist remains the same. If a majority of the miners colluded to exclude an address, there are some client side ways of giving favor to the "honest" miners, assuming that a majority of the client nodes were willing to upgrade to a version that protected again this.
UPDATE 2021: (8 years later)
There's been some activity on this Q/A recently that brought it back to my attention. And I wanted to note that this scenario effectively did happen in 2016 for Etherium. Through a weakness in the DAO (decentralized organization) protocol, a huge amount of Ethereum was stolen. The amount stolen was a enough that it effectively jeopardized Etherum's viability.
As a result, a hard fork was proposed. In other words all of the miners colluded to prevent any money from being removed from that account exactly as proposed in this Question. It's actually slightly more complicated and extreme than that: essentially all blocks after the hack were invalidated via collusion between the miners.
Obviously this Question was about Bitcoin, not Ethereum, but it is a real world proof of the answer. Can a coalition of miners "steal" money from an account by locking it down? Absolutely: in the end a coalition of miners with a supermajority can do almost anything. In this case, a supermajority of miners "stole" money out of the DAO account because the funds had been obtained via a protocol bug. Which is exactly why it won't happen in anything but the most extreme circumstances (such as a bug in the protocol, like the Etherium DAO incident) because it wouldn't be in the miners self-interest otherwise.
More info on the Etherium DAO incident here.