I type a lot, so not hurty wrists is a big thing for me. I’ve been very very happy with the Microsoft Natural Ergonomic 4000 Keyboard (available for as low as $30 from time to time, and sometimes on sale that low at Staples or whatever). And it works awesome with the Mac, too.
The best part of it is not just the split design, but that it’s also tilted backwards at a negative angle, which is perfect for people like me who are lazy and have never learned to keep your palms UP, but instead put them on the rest.
One drawback though is that the plastic piece that keeps it at that wedge eventually breaks, it’s got three small plastic spots where it attaches and those have snapped over time (nearly 2 years now, I think). The keyboard still works awesomely, it’s just the slope that was gone.
So, rather than dump the keyboard, I did some digging and found the VersaTech Keyboard Wedge, which is a piece of thick, tough foam cut at a 10degree angle, and big enough for the keyboard. It’s a little pricy ($40 bucks shipped, more than a replacement keyboard) but this foam isn’t going anywhere, and my keyboard is nice and negatively sloped again.
This wedge would also work well with your existing keyboard, giving it a perfect negative slope, if you don’t want to buy the MS product for some reason.