There have been rumors of LED backlights coming to Macs for awhile now, but as usual Apple remains mum on future products. However, we did get a rare glimpse into the future of the Mac via Steve's open letter called 'A Greener Apple.' He discusses the use of arsenic and mercury in LCDs and mentions that Apple plans to ship the first Macs with LED backlights in 2007 (which use no mercury).
There you have it, straight from the Steve's mouth. The only question remains which Macs will get the LED love first.