We use Google accounts as the single login for LittleData - mainly because we need access to Google Analytics API.
Including Google Apps, there are probably 1bn+ Google accounts
What we've found is that business users are often confused by Google's account system - they have multiple Gmail account set up for accessing different parts of Google's ecosystem, and can't remember the passwords for all of them.
It works much for smoothly for organisations that use Google Apps, since their main email is also their Gmail account.
So it's not pain free, but I understand the need to hook into Google's oauth system. As an aside, if you want to achieve the same collaborative editing features you could look at the Meteor framework - although I think Realtime API may beat it on conflict resolution features.