I have both an iPad 2 and 3, and I'd like to know if anyone else can confirm the differences I'm seeing (and am not particularly happy with).
The iPad 2 has a perfectly white background, but the iPad 3 is off-white/yellowish. Most noticeable when reading e-books, e.g. in GoodReader, Kindle, etc.
The brightness is 100% uniform on my iPad 2, but I can detect a very slight imbalance between the left and right sides in portrait mode on the iPad 3. The problem follows the same screen side when I rotate the unit.
And for a listen-and-compare issue, I have identified that the iPad 3 has a lower volume than the iPad 2 (based on my tests it's ~ 5-10dB less).