I switched from XP to Ubuntu 8.04 a few days ago and I haven't been able to get suspend to work. I have an ASUS P4P800SE motherboard with an Intel P4, 1 Gb of ram and a Radeon 9550GE video card.
When I try to suspend, the computer hangs with the hd light on. I have to cycle the power to reboot. I logged the output of sleep.sh and it ends with "echo -n mem". I checked kern.log and the last line before reboot is:
"[ 2874.186977] ACPI: PCI interrupt for device 0000:02:05.0 disabled".
I tried with linux 2.6.24-18 and -16 and get the same results.