Comment 7 for bug 193323

This might also give some clue on why it worked with gutsy, and why it doesn't work in hardy:
"(II) CIRRUS(0): Creating default Display subsection in Screen section
 "Default Screen" for depth/fbbpp 24/32
(==) CIRRUS(0): Depth 24, (==) framebuffer bpp 32"

in gutsy, the framebuffer was still with 24 bits.

My guess what's going on there is the following:
xserver-xorg-video-cirrus will do in src/alp_driver.c an initialisation (function AlpPreInit). In there (somewhere around line 960) the maximum dot clock frequency gets set, depending on the bits per pixel (these seem to come from the framebuffer bpp?). For the gd5446 that's 0 though (line 146), as it doesn't support that 32bpp.

This still doesn't give much clue of *where* this comes from though.