This sounds like a nice change for us to consider. I see your point regarding hosting your site on multiple domains and can see how your patch would fix that.
One thing I wonder about, though, is how well this would work with some providers (OAuth 2-based providers, specifically) where the callback URI is verified against the registered application. The latest draft of the OAuth 2 spec says "The authorization server SHOULD require the client to pre-register their redirection URI or at least certain components such as the scheme, host, port and path. If a redirection URI was registered, the authorization server MUST compare any redirection URI received at the authorization endpoint with the registered URI." Gowalla, for example, requires that you preregister the callback URL. Facebook allows for subdomains, but requires that the callback match the main domain. In contrast, Twitter allows you to register multiple domains, specifically for the case you have.
Nevertheless, it's a good thing to consider for Spring Social. May I suggest that you submit a pull request in GitHub for this change? If you're unsure of how to do this, there are instructions at https://github.com/SpringSource/spri...-Spring-Social to help.
Spring Social Project Lead