So I've had problems with my droid (2.1) randomly deleting my texts into oblivion. Sometimes there are many MMS messages in my texts when this happens.
After a long period of time, I found "dialer storage" taking up 170MB of space, which was cramping me badly but I couldn't do anything about it.
It appears that all of those MMS's that "disappeared" were still hogging space.
In 2.2, I just went and found dialer storage (170mb) and deleted data. I now have 200MB free on the phone!! w00t!
Hopefully it'll stop randomly deleting messages, too. One can only hope.