If you click show source on a message it pops up a new window with the source. However it's being sent in content-type of text/html which means it's all one one line and you can't read it too well. It should be in text/plain shouldn't it like on old releases of roundcube?
This is on the new 0.1-stable release.
I also get these errors in the log:
[04-Mar-2008 13:30:36] PHP Warning: session_start() [
function.session-start]: Cannot send session cache limiter - headers already sent (output started at /usr/home/matt/www/roundcube/config/main.inc.php:285) in /usr/home/matt/www/roundcube/program/include/main.inc on line 83
[04-Mar-2008 13:30:36] PHP Warning: Cannot modify header information - headers already sent by (output started at /usr/home/matt/www/roundcube/config/main.inc.php:285) in /usr/home/matt/www/roundcube/program/steps/mail/viewsource.inc on line 26