I too was "forgotten" a couple of times yesterday, and had to log back in, but it seems OK now.
This happened about the same time as I lost all my Address Book data, Address Book was suddenly completely empty. I have now idea if I accidently deleted them myself (I was washing the screen and keyboard at the time) or if there was a hiccup that wiped the Address Book data, but combined with being booted out of MR i feared that OS X was on the brink of a severe system failure, and thus got a bit nervous, not to say on the verge of a complete breakdown...
But, since I had backup of my Address Book data, just a couple of days old, that was easy to restore, and reading about others also having trouble staying logged in at MR, I soon calmed down.
It's nasty when two similar, but really entirely unrelated, accidents happen at the same time.