It seems as though with every release of OS X a common and persistent problem experienced by Mac users is their machines to go to sleep automatically after a certain period of inactivity. From my current experience OS X 10.5.2 is no different in this respect to any previous release. As usual there are a number of threads on Apples discussion boards and on other Mac forums.
I have listed a few of the threads below for information. If anyone reading this has successfully fixed their sleep settings I would love to hear what snake oil and incantations you had to use to perform the miracle !
http://discussions.apple.com/thread.jspa?threadID=1394763&tstart=0 - 10.5.2 combo update - auto sleep no longer works. Interestingly the suggestions in this thread of running pmset -a fixed one of my two iMacs but not the other ! They are both running 10.5.2 and are new Intel iMacs.
http://forums.macrumors.com/showthread.php?t=378085&page= - MacRumors - Leopard will not sleep ?
http://www.lockergnome.com/osx/2006/11/03/why-does-one-mac-play-while-asleep-and-another-wont-ever-go-to-sleep/ - OS X Fanatics - Why Does One Mac Play While Asleep And Another Won’t Ever Go to Sleep?
UKMac.net - Switching the World to Macs one person at a time !
My Mac never seems to stay asleep - I often put it to sleep before work then when I come back its awake again!