I am having a similar problem and it may be my own fault because i wanted my single player world to become a server.
First I was getting in and crashing right away.
So I took my SSP world and converted to to 2.1.1 with the converter thing and put it in my SMP world. I can run around for about 20 seconds or so and then it crashes. I have been tinkering with different settings for a while now and i can now log in for about 20 seconds before crashing.
I have tried changing the java from 7 to 6 and back and it made no difference.
I messed with the Ram settings.
I have tried fiddling with the actual map stuff itself to no avail.
No matter what I do it will log in and pretend its working then crash after a bit. No specific thing in the game causes it to crash (at first i thought opening a chest or falling).
(same settings now)
launcher version - 0.5.4.6
operating system - windows 7
java version - 7
server log from when he crashed - [iNFO] Connection reset
[iNFO] ancientflame lost connection: disconnect.endOfStream