I'm basing it on one of H20's last posts here...
The US server now has the new code, which doesn't disconnect on coin switches. It's bugged to have 1% extra rejects, which show up as "duplicate shares".
Of course that could also mean he subsequently pushed that code to the the non-beta server too...but why do that if he was going to replace middlecoin.com with the front-server that would switch you to your regional server eventually anyway. I know I wouldn't change the existing stable server until all the beta's are done and then I would update it. But you're right, that part is definitely an assumption. H20 hasn't said either way.
But USEast (and EU and Asia) are definitely on a new and different codebase than what was there in December.
I read these two posts as all servers are running the same code base and the 1% rejects have been fixed.