Use OpenID, perhaps? Add a ClickPass widget?
If that's the case, it speeds up the registration process by a lot, and _nearly_ replaces it. (not quite since I'd need a username)
I think many sites require extranenous registration information just because other sites request the same information. It also gives them the option of analysis at a later date.
Some companies may actually use the information they collect for data mining or to resell. Even though this can sometimes have evil purposes, sometimes it can be beneficial for a company to say Paul Graham really likes chocolate chip cookie dough ice cream and its his birthday, but for most places collecting this information is of little use. It's just annoying for the user, as the many replies to this news posting can attest to.