A unique alias is fine in circumstances such as the one you've given--my point is about account creation and logins. There is no reason you can't use an email as a login and then require the user to give a unique alias to use in their URL. FogBugz (On Demand), to cite one example, does things this way. You login with an email but when you sign up, you have to give a unique corporation name and the app generates a subdomain of fogbuz.com; e.g., mycorp.fogbugz.com.

Anyway, I really like the idea of what you're doing, but as time marches on I am becoming increasingly zealous over this issue. Retooling your app at this point is probably too much work given that most users don't have the same conviction as I do but since you asked for feedback, I'm giving you my 2¢.