It would have to do wih ACPI. If not needing any ACPI then you can issue the command as root ' /sbin/service acpid stop '
Now hit the sleep key to see if it sleeps.
If that does the trick you can do a few things.
Leave it off by issuing the command ' /sbin/chkconfig --level 345 acpid off '
Or goto /etc/acpi/events and rename sleep to sleep_backup or move it to say to /etc ' mv /etc/acpi/events/sleep /etc '
A few other ways but more complex.
Lets see if that stops it from happening first.
Distribution: RHEL 5.1 with Pieces of this and that.
Kernel 22.214.171.124, KDE 3.5.8 and KDE 4.0 beta, Plus Development src.rpm, ATI fglrx64_7_1_0-8.433-1 rpm with 3D and DRI working.
Acer 5100-5840 with webcam, ati, sdcard reader, sound, atheros based wireless, all working. Only thing not working is the memory stick reader.