I found that with Ubuntu 9.04 x86-64 disabling compiz/beryl(desktop effects in GNOME -- not sure if xfce wm supports that as I haven't run an xfce is some time(was testing it as a more featured replacement for fluxbox on an old nb)) solved the freezing on wake from sleep/hibernate for me...
It'd be really nice if 9.10 final actually fixes that, along with video playback also occassionally freezing the machine after wake from sleep. (It doesn't happen all of the time now, so they must've at least made a partial fix for it so far...)
(4850 mobility 512MB GDDR3, P8600)
Intel Core 2 Duo P8600, 4GB DDR2-800, 320GB 7200RPM WDC WD3200BEKT-22F3T0, ATI Mobility Radeon HD 4850 512MB GDDR3, 8x Super multi DVD+/-RW, 1680x1050 (17"), 9 cell battery
Ubuntu 8.10 x86-64 (current updates) catalyst 9.3
Windows Vista Home Premium 32b SP1 (current update) still on shipped catalyst(8.12 I think, MSI packed -- lazy)