Right, I was able to test mIRC under Vista with aero glass, however I was not able to reproduce the issue you mention. When I run mIRC under Vista Aero Glass, the mIRC titlebar is translucent as expected. I then maximize windows inside of mIRC, eg. the status window and the mIRC titlebar is still translucent.

Are you able to reproduce this with a new cleanly installed copy of mIRC, and just by maximizing the status window? Or is it only happening in other situations?