Do not use any builds from November 19th, 20th, or early part of the 21st - it can destroy irrecoverably any armature file used.
Also, please delete any builds you are personally providing to the community from these dates.
All test builds made between 2006/11/19 00:07:32 CET and now will mess up Armatures, if you enter editmode once. That's an unrecoverable error, so you might consider to remove such testbuilds to prevent damage to animation files.
Info obtained from the developers maillist.