I think LED on the N900 will probably last you 100000hours of continuously lit 24/7. My samsung LED TV last 100000hours as well with about 7-11years usage. So I guess as long you did not overpowered the damn thing, then it will not break within the next few month.

Not sure about the LED on the iPhone4 though. Apple using the cheapy LED and no hard detail specification of it. Nokia fully documented the N900 hardware and that make it safer than not knowing the limitation