The new ipad has an excellent resolution, but there are tradeoffs - cost/power/battery life. I think the resolution is higher than optimal for current hardware given the processing requirements.
So I agree that cleartype has a shelf life, but it will still be valuable for quite a few years.
It will no longer be necessary when screen resolutions (pixels/inch) double (will take 10 years at a guess, going by historical exponential rates), or if OLED takes over (noone will bother to write new algorithms for different subpixel patterns).
For the lifetime of Windows 8 it will still be very valuable - except on the highest end devices. And so it's a major omission for Metro to leave it out.