iOS and OS X have been using AA for text for a very long time (10 years). The catch is that the AA algorithm that Apple uses is different than the one that say, Microsoft uses in Windows. ClearType aims to produce crisp text, even if it slightly distorts the typeface used, and Microsoft picks default fonts in Windows based on minimizing that distortion effect. Apple's AA algorithm aims at more accurate typeface display, even if it winds up a bit fuzzy as a result. The catch is that with a PPI only in the 130 PPI range, there aren't enough pixels to make a typeface look crisp, and Apple likes to use curvy fonts that require AA to look right, producing the fuzzy text on the iPad.
Without the AA, the iPhone 4's display of text would not look anywhere near as good as it does.