Good write up for a sensible solution to a problem which should effect lots of sites.

Personally, I would probably take the approach of settings a cookie per user. This would be very suitable when beta testing is enabled/decided by users, rather than a subset being chosen A/B style based on a value in the database. This would have the advantage that nginx would not have to make any database requests; just check a cookie on the request, likely speeding up the speed at which nginx handles requests. However, it would require some changes to the application (versus [small] changes to the database which the slug approach) to set the cookie etc.

I wonder how easy this is to do in apache.