I’m having the same problem since upgrading to 2.9.1 on two of my wordpress sites that I’ve been active on:
bwwp.net
nibblesandvittles.com
I can visit one minute and the posts are there, the other minute it’s either gone or reverted back to a revision from a week ago with no trace of the newer version in revisions. I use the ‘wordpress database backup’ plugin and I guess the new version was absent during my backup today as it wasn’t included in the SQL file.
I’m concerned that I’m going to permanently lose work and it seems this is not an isolated incident.
I run several other WP sites that aren’t currently very active but haven’t noticed any problems on those, although I haven’t dug very deep, some would still be running WP 2.9, most on 2.9.1