There are 3 versions of the Ultra X:
1. The Logitech Ultra X PS/2 Keyboard
2. The Logitech Ultra X Wireless USB Keyboard/Mouse Combo
3. The Logitech Ultra X Media USB Keyboard
I do not have #2, so this First Looks will cover only #1 and #3
Here it appears that there is nothing really different, however there are a few subtle changes to this model.
As you can see here, Logitech opted to use two black strips (very glossy = fingerprint magnet warning!) on the top and bottom, and there is more of a beveled edge around the keyboard rather than a straight edge on the sides as compared to the previous model.
Another thing to note here are the keycaps. They are slightly more curved. As for typing on the keyboard, it has a slightly more cushioned feel to it, similar to other Logitechs (similar to Apple keyboards). I am not really in a position to pry one of my keycaps off to see what kind of mechanism it uses, but I believe that the older model uses a mechanism that is much more tactile (I type faster on the older one than I do on the newer one).
Both of the keyboards have the same media keys, by the way, and still are natively supported by Windows XP (requires no software).