It's definitely a feature many users would like to have. After all, Facebook's biggest competitor MySpace lets you choose any available URL for your profile, in the form of "myspace.com/name". All Facebook sees this as a possibly very lucrative business for Facebook, if we put bidding for names into the mix.
Would I pay for it? No. But it doesn't mean that Facebook won't make money out of it.
Vanity URLs are a trivially simple feature that the competition already has, and that should have been implemented ages ago. On the other hand, they're
Facebook is obviously looking at charging for a premium service, akin to Flickr. Vanity URLs could be one part of it; if Facebook were to charge for them alone, I'd find it slightly insulting, but within a mix of other premium features, it could be worth paying a small fee.