This has been driving my crazy for a while. Whenever I switch from MetaCity to Beryl and open tsclient, which ultimately loads rdesktop, the window is translucent and unusable when connecting to the remote server with the colours set at anything other than 8bit. Today I figured I'd Google it. Low and behold, the Gentoo Wiki had the answer:

Open a console and type:
$ export XLIB_SKIP_ARGB_VISUALS=1

Then run the app from there or prepend "XLIB_SKIP_ARGB_VISUALS=1" before the command.

Source: http://gentoo-wiki.com/HOWTO_XGL/Troubleshooting#...translucent