I too wish someone would create a pad like this. It could probably work in two pieces that clip onto the left and right side, so that you can still operate the touchscreen and more quickly remove it if you suddenly get a phone call.

The Wiimote is good, but only if you are sat down at a desk to prop the N900 with the kickstand. This however would turn it into a full handheld games system, and one a lot more comfortable than the PSP or DS I suspect as its an ergonomic pad shape (which those handhelds sadly are not).