After I updated from Joomla 1.5RC2 to 1.5.2 stable, just copied all the new files, I had the problem that all images in an article in a submen were missing when I turned on SEF. It turned out, that a lot of other people had the same or similar problems.

It took my a while to find the correct solution but in the end the solution was easy in my case. All you have to do is APPLY the SQL statements that are supplied in the installation directory. Those that are namend diff_*. In my case it was "diff_rc2_to_rc3.sql" and "diff_rc3_to_rc4.sql".

Full thread on Joomla! Forum with further explanation.