1.5.1 Crashing a Few Moments After Loading Survival

Since upgrading to 1.5.1 Minecraft will load, and then crash after a few seconds (using survival mode; haven't tried other modes). The crash seems to be triggered by hitting Esc (to get to the menu), or E for inventory, once the survival world has loaded. I can walk around for a bit, and the game runs fine, until hitting a button to load one of those menus.

Help would be appreciated - thanks.