You could go into Settings > Applications and clear out the data on some of the apps. When you go to Settings > SD Card & Phone Storage you can see the disk usage and see whether there is sufficient storage available.
Before clearing the data, I would ensure that you have saved any data you want to retain (example: twitter, all locally stored tweets will be removed, but once you hit refresh again in the app, the latest tweets will be saved)
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Dreaded "Text Message Memory Full"
OFrogdroidianB says,
Has anyone had the dreaded "Text Message Memory Full" Alert problem? Even more important, does anyone know of any good solutions to the problem? I have SMS Backup+ and have been erasing all of my texts after I read them now and have recently reviewed my installed apps and removed some of the ones that I have not used in a while or ones that I was not extremely happy with, but this is the 2nd or 3rd time I have gotten this message since getting the droid. I have also moved all apps to the sd card that I could. Any suggestions for dealing with this issue would be very helpful.