One of the more recent third party solutions is the Kiano 4 iPhone keyboard. It's available in your choice of black or white and, well, it effectively turns your iPhone 4 into a slider phone with a full keyboard. It doesn't make a wired connection to the iPhone, however, relying on Bluetooth 2.0 instead.
Yeah, that's going to cause some battery concerns, not to mention that you'll need to charge it separately from your smartphone too. I guess there's a purpose, but if you really must have a keyboard, you're probably better off ditching the iDevice in favor of something like the HTC Desire Z instead. In any case, the Kiano4 is $99 from some random HK e-shop.