Actually there are several ways of looking at this, with regards to moving parts the FAN is typically the only part that is moving right, and if this has issues you should know about it through lack of noise.
With regards to leaving a unit on the generally a machine that is left in a perminant state SHOULD theoretically last longer than one that is turned on and off each day, every time you turn your computer an electrical current passes through its components, expantion and wear and tear occur.
However if you are a home user I suspect the cost of replacing a PSU is out weighed by the cost of your electricity bill.
AMD 4200+ X2 OC 2.3Ghz
Club Nvidia 7800 GTX 256megs OC 540/1350
Asus A832n Delux MB
40 gig WD Raptor
300 gig WD Sata 2
3dmark2005 = 8652