Since we've already asked this question, let's try to re-answer it with some additional insights:
Without mining or relaying, in order to participate in the network, one would need enough time to download all transactions within a block before the next block is found in order to stay synced with the network. Considering that a block has a maximum size of 1 Megabyte and that the block time is about 10 minutes, this puts the theoretical minimum download speed at about 1 Megabyte per 10 minutes or 1.7kbps (according to Google)-- much slower than a dial up modem. Notice that this figure is constant -- so the size of network does not dictate how much bandwidth is required to participate in the network, so this was true in 2011 and will be true in the future.
In the original question, we wanted to run a wallet from work. If all we want to do is trade bitcoin, this should be fine (provided that you do download the initial values for the block chain outside of work), but to do more on the network...
If we're relaying transactions, not just downloading them, the calculations can vary wildly. Here our bandwidth should be proportional to the number of transactions posted per second (which should be predictable) as well as the number of node to which you are connected (which isn't easily predictable).
If we're mining, you're going to have to broadcast your mined blocks, which as mentioned before, can have a maximum size of 1 MB, and you're going to want to broadcast this as fast possible -- not just within the 10 minutes. You could put the theoretical minimum required upload speed at 1.7kbps, but it's simply not practical.
I realize that this doesn't the question about how much average-bandwidth clients us, but if you're only posting and reading transactions, we shouldn't need more than about 1.7kbps. If you begin mining and relaying, we're going to need more, but it's hard to predict how much.