First, we have a whole bunch of threads on this, so a search would help, and it would offer dozens of answers to this you might want to think about including a long thread where I reviewed a whole bunch of keyboards for the ipad3....
But in short,
The Zagg Folio fits both the 2 and the 3 -- most people are finding in general that almost all cases made for the ipad 2 work with the iPad 3....
The Zagg has what I have found to be, by far, the best keyboard with the least errors -- in fact, I was able to get about 60 wpm on the keyboard without error after only a few minutes of practice. The downside is that the case is flimsy, looks kinda cheap, and its really hard to get the iPad out of the folio case -- I've found I have stopped using the folio at all, and just use the keyboard on my desk at work.
The clamcase fits both the ipad 2 and the iPad 3 -- it is by far the best overall case/keyboard that I have found. It looks great -- and the keyboard works well. Because of the realignment of the magnets in the ipad 3, it does not turn on/turn off the iPad like it does with the ipad 2, where that works flawlessly.
The downside is that the keys are cramped, and the error rate increases. With about a half hour of practice, I got about a 45 wmp typing rate, and that included several errors that required correction. It's a tradeoff because the keyboard feels good, the response is good, but because the keys are not seperated it is harder to type. I use this case only when I go on longer trips and don't want to take my laptop.
The nest overall keyboard is the Apple bluetooth keyboard without a case -- but it's a pain to lug it along, and you need a seperate "easel" for your ipad. Still, it's the best keyboard you can use -- and I get 65 wpm error free with this keyboard. I have one at home, and it's my keyboard of choice when you don't need the case.
Finally, I've gound that Brookstone makes a decent keyboard -- it works well, comes in multiple colors, and its actually more stable than the Zagg folio. The keyboard is not as good, though, and is plastic-y feeling. Still, it might be a fair tradeoff.
Almost all other keyboards are made by the same companies and distributed to third parties who put them into their own cases. I'd stay away from most of the other keyboards.
Overall -- for the convenience of the sheer folio thing, the Clamcase is about as good as it gets, even though it is about 50 dollars more than the Zagg and other similar models. Plus it makes your iPad look like a macbook, which is kind of cool.