That's in large part what WebFinger was created for, which I used as a model when developing this project: http://hueniverse.com/webfinger/
Gmail actually used to support it to a degree (it came along with Buzz) but in September I think they removed the host-meta file. WebFinger is still a big part of OStatus (which is what identi.ca is based on).
The main issue I have with WebFinger is that unless you have your own domain name, it's either tied to an e-mail service, or you get an identity that looks like an e-mail address but isn't. Gmail had it, then dropped it, and given Google+ I don't think they would have too much interest in bringing it back. OStatus acct: identifiers are a lot like e-mail addresses but I don't think they function as such.
Of course if you have your own domain you can make it so that isn't the case, but most people don't and if you try to create a free domain that makes it easy for people to do that sort of thing you basically end up with aka.nu anyway.
