There's this idea I've had for a while, but never got around to implementing: every namespace is a market. So there should be a place where you can trade ownership of tokens in different namespaces. There already exists a healthy market for domain names, although it is heavily fragmented, but usernames on websites are becoming valuable enough that it makes sense to trade usernames as well. Once such an infrastructure exists, you can imagine auctioning a variety of things this way -- phone numbers, license plates..
I've fleshed out the idea in my head in some detail, but that's the gist of it.
I think the reason for this is that it might be a little strenuous on the client's JS processor to do that (sure not for up-to-speed browsers on decent PCs - but usrers on slower systems may encounter some sluggishness). Also there's some server issues to be taken into account.
I wouldn't be surprised however if later on they decide to deploy advertisements - making the user stay on the page for longer is a good tactic to try and get them to click on an advert.
As far as I know it does do some caching (or at least I've heard @usernamecheck (http://www.twitter.com/usernamecheck) tweet something to do with caching).
A natural feature for this would be to wait until the user leaves your site, then register accounts at all those sites using the supplied username. Then squat on those new accounts and offer to sell them back to people.
The utility of a consistent username across sites that have them visible (usernames are obviously more useful at flickr than at your bank, outside of ease of remembering) is mitigated by content aggregation sites like friendfeed. Once I've added a service to my "Me" listing at friendfeed, it becomes authenticated to my identity, no matter what the exact username is.
A privacy statement for a usually public username anyways? I don't think this kind of stuff needs to be done client-side only. If passwords were being sent along too that would be something different but usernames are meant to be shared and usually somewhat public information.
Unless you're worried about the maker of the site "stealing" your identity on some site... although thats a pretty weak argument. If there is a "mitchellh" on some site I just make a "mitchellh3" or something of the like... no big deal.
Very clever little tool. I can't see myself using it too often, but it's shown me lots of semi-popular sites where I can still pick up my username.. so I might just have to do that next!
I've fleshed out the idea in my head in some detail, but that's the gist of it.