no clock, just a white square

This problem you see is not related to cairo-clock, but the xserver and compiz. What you are seeing is client (compiz) and server (well the xserver) disagreeing about the OpenGL texture-target to use for drawing the redirected window (the one of cairo-clock). One of them wants a power-of-two texture-target the other one a non-power-of-two texture-target and in the end the user just sees a white rectangle. This issue in the xserver if fixed upstream. For the related bug-entry in the xserver see...

  https://bugs.freedesktop.org/show_bug.cgi?id=11109

Until xserver 1.4 hits the Ubuntu repositories just make sure you use cairo-clock with non-power-of-two width/height to avoid this "white rectangle"-thing. Do achieve this you can do two things...

1.) Start cairo-clock from the command-line like...

    cairo-clock --width 200 --height 200 (and any other options you like)

Just don't use any power-of-two value for width and height (e.g. 64, 128, 256 etc.).

2.) Or while having started cairo-clock form the applications-menu, right-click on the "white rectangle"-window and select "Properties". Change the width and height via the appearing dialog.

Could the default size of the clock be updated so that it was not a power of 2 thereby reducing bug reports about cairo-clock in Gutsy?