I too had this problem with an old Compaq notebook (a 486, I think...) - one corner of the screen went very dark, with a weird-coloured fade towards it, and was much warmer than the rest of the screen. It turned out it was a failed pixel that had fused the matrix and caused lots of the surrounding area to overheat and black out.
I have no idea if this is still an issue, I think this may be a passive matrix problem, but if this information is any use, get down to Apple Store or if, like me, you're in the UK, use your AppleCare...
: P