Why is the screen saver not kicking in?
I cannot tell about screensaver for sure, but you can use pmset -g to show which process is disturbing sleep (think this first appeared in 10.6.7)

# pmset -g
[cut]
sleep       0 (imposed by 1517)
[cut]


this means that pid 1517 is preventing the system from sleeping.

# ps ax |grep 1517
1517   ??  Rs     1:59.76 /System/Library/CoreServices/backupd 


This shows that this is the backup daemon running.