Lately I've had a couple users report that they will see mail show up on their device, and then disappear later on. I've seen this myself on a user's phone. I tried removing the account from the device, rsyncing the device, rsyncing the user, removing the user and re-adding - none of this seems to work. Not happening on everyone as far as I know, I know it's not happening on my device. We all use the same device - Droid Incredible.