BEFORE upgrading to 1.7, are you using the SocialRewriter filter in web.config?
Disable this by adding <!-- and -->to either side of the tag.
If you haven't done this and now have a website displaying "Server error in "/" application" or similar - don't panic! Just get into the web.config and add it.
I also disabled custom skin additions relating to Active Social welcome view, but this may not be necessary.
Now going to check if there's something in the release notes about this, or otherwise raise a support request to figure out how to get my vanity url's back :-)