Same here. I've managed to dig in a little deeper. Trying to find out what is going wrong. RC1, does not work at all. The messages are not written into the database. HOWEVER: I've installed roundcubemail-trunk-r675-20070813, and it does not work exactly the way it should, but it's improved. The mailheader now shows, without a problem.
When selecting an e-mail and clicking directly on reply, the original record in the database stays intact, and therefore reply-ing WORKS. When double clicking on an e-mail, which shows the content the record in the database is overwritten with empty fields.
So in my opinion there is an incorrect (update) SQL statement somewhere?? in the program.
I've also found some messagesize bugs and time-date bugs. Should I post them?