Sure enough, three bugs were reported immediately. (And each of them a few hundred times, no less.) We do test this stuff for quite awhile before putting it live, but a half million testers is always better than 20.
Anyway, bugs are fixed, and as the first one was serious, we logged everybody out again.
That's why it seems that you can't stay logged in.
Sorry, things should stabilize soon here. There are lots of advantages to the new login system which make this half hour of pain worth it.