After the new, third-gen iPad was announced, Apple dropped the price of the previous generation iPad by $100 (for every version), like it normally does with its iPad and iPhone lines. However, Apple also changed the system-on-a-chip CPU that the iPad 2 came with. With no difference in the exterior, you won't be able to say whether your newly-purchased iPad 2 is the newer version - 2,4 - or one of the old ones.
The iPad 2,4 model comes with a slightly re-engineered A5 processor that it originally shipped with. It now comes with a 32nm chip instead of the old, 45nm A5.
This slightly newer chip is slightly more efficient, giving the iPad 2 slightly better battery life, that Apple hasn't even talked about. The guys over at AnandTech did some tests to find out more about it and also wrote an amazing article explaining the new chip for the geeks.
They report that they gained 15% battery life when web browsing, 30% more gaming battery life and an 18% gain for video playback.
The numbers are astounding. The iPad 2,4 has a significantly better battery life than the old iPad 2s. More importantly, it has a better-than-advertised battery, which even beats the third-gen iPad.
Now you ask, where can I get one of these? Well, the problem is, Apple still has stock of old iPad 2s left, which it continues to sell. There is absolutely no way of telling the version you're getting without opening the box and checking it through software. So you just have to keep fingers crossed.