FYI, I have a rooted RAZR.
I use the AndFTP app to transfer files to/from my computer wirelessly. I don't know if the AndFTP app is the culprit with this issue, but that's usually what I'm using when I notice the problem.
Sometimes when I try to download files to the phone, I get an error that tells me it's a read-only file system. This may happen even immediately after already downloading some files. Once this happens, I can't seem to modify any files on the sdcard in any way until I reboot the phone. Using any file manager, attempts to delete/change files result in error messages such as "cannot rename file" or "file cannot be deleted".
So far, the only solution I've found is to reboot the phone completely. I'm not sure if there's a way to actually prevent the issue (which might be caused by AndFTP), but I would like is to at least find a way to restore the file system's write permissions without having to reboot the phone.