I have this problem too, i can see the summary of the message in the conversations window but when i open one to view it, its empty. This applies to all types of messages: SMS, IM etc.

It appeared after i did an OTA update to PR1.3. I then needed to reflash anyway to fix a different issue but this one was not fixed.

I think its similar to this problem reported a while ago with no resolution: http://talk.maemo.org/showthread.php?t=55198&page=2


Edit:

I have found the solution in Bug 11456 by renaming the rtcom directory in the user folder.