I got a regression for you here...
Ubuntu 64, 16gb, 2* gtx560 ti oc 2gb each, i7 4th gen
Broken: newest version from trunk
Worked: Official 2.70
Short description of error
I couldn't get any rigid body constraints to work. Oftentimes the rigid bodies just disappear, sometimes they act completely weird like bouncing back and forth at a very high speed. Tried it with 2.7 and all is working well there.
But this is only happening when I build my version with the "Release" option set. On "Debug", everything works as expected. I tried it also with a downloaded buildbot build, but as those are compiled with "Release" too all is working there as well.
Exact steps for others to reproduce the error
I'll append a file from Zuggamaster that is hosted on blendswap http://www.blendswap.com/blends/view/72240
Just start the animation and you'll see the issue.
File is here: http://www.pasteall.org/blend/28278
As we like to release an "a" release - this should definitely be fixed for it.
Many greetings, Thomas