I have the same problem. I was using Ubuntu 10.04 (lucid) with the xorg-edgers PPA, so when it upgraded libcairo2 from 1.8.x to 1.9.x Rainlendar started black. Now in Ubuntu 10.10 (maverick) with the default libcairo2 1.9.14 Rainlendar has black windows.
With the Kwin composition Rainlendar windows are transparent.
Can the package libcairomm 1.8 be the problem? Maybe it has to be in version 1.9
Tested with the latest stable (b91), beta b85, and beta b94 from Rainlendar.
aMule (also wxwidgets app) works ok.