Given the difficult of detecting lost coins, or telling between hoarded and lost coins, it seems impossible to actually report on the real wealth distribution of the bitcoin currency.
The protocol gives no information about who controls an address. In an extreme case, every address could be controlled by just one person, or every address could have a unique owner. However, a degree of traffic analysis is possible, and if you have access to other data from outside the Bitcoin network, you could correlate transactions with these non-Bitcoin events and infer who the real-world owners of the addresses are.
For example, if you had access to ISP records of who uses an IP address that has just mined a block, you could start to build profiles of the owners of new coins.
But there are many barriers to achieving the total information awareness that you would need to build a comprehensive profile of all coin owners, so the short answer is no, there is no reliable way to measure Bitcoin wealth distribution.
Well, we know at least how much Mt. Gox holds -- as they have a cold storage system that they've proven they control.
Also, flow analysis combined with other data might help make it possible to have a better idea. It alone doesn't give exact identity, but it can help to point in a general direction as far as how much certain people or certain organizations have received and of that amount, how much they've spent.
Here's an article describing this: