Mark Rein, the vice president and co-founder of Epic Games, one of the most prominent game developers for the iOS platform, and one whose lead iOS game, Infinity Blade, was used during the launch of the iPad 2 (and the iOS 4.3), let slip two very interesting titbits during an interview with the Seattle Times.
Rein said that "the CPU [central processing unit] and GPU [graphics processing unit] in the iPad were introduced before the iPhone 4, so the iPhone 4's more powerful and has more features.". Further in the interview he said that "This [the iPad 2] is now more powerful than the first-generation Xbox. This is probably more powerful than a PlayStation 2 or a PlayStation 1 for sure. This is on the road to that, if it's not already."
The first quote means that Apple has developed a different version of the A4 for the iPhone 4, one with more features and which, according to Rein, is more powerful. Apple never disclosed the exact specifications of the A4, only saying that the same chip was used in both the iPhone 4 and the iPad.
The other quote about the graphics capability of the iPad 2 compared to the PS2 is a very powerful statement but one thing that has to be remembered is that the Sony Playstation Portable had been through there and the Sony Next Generation gaming console is likely to be much better, graphics wise, than the iPad 2 when it launches later this year.