If you’ve just upgraded to the new OS X system 10.5.7 you might experience some problems. One of them is a system crash whenever your computer goes to sleep. Fortunately, right before I was going to revert my MacBook back to 10.5.6 with my Timemachine backup I found the workaround:
Look at the settings in your active network environment. If there is only ‘Airport’, your Mac will crash every second time it enters sleep mode. Add ‘Ethernet’ in your active network environment. Now it should work.
Update: Frank just checked on his brand new MacBook and it doesn’t happen there. So all I can say is it is happening on my MacBook 2.2GHz.