Just because the Xbox360 Chatpad wasn’t designed to work with the PSP doesn’t mean that some creative and skilled people can’t make it happen! The PSP Phat and Slim Keyboard is proof of that.
"Hey all talk a look at my Microsoft Xbox360 Chatpad connected to my PSP Phat and Slim. Thanks to Jean over at ps2dev.org and this project that he developed we can all have nice keyboards connected to our PSP’s. My PSP’s are running CFW 4.01 M33-2, a special version of piKey and a custom flash on the Chatpad. Text input is a breeze and it works with the Sony OSK (on screen keyboard) and the Danzeff OSK under AFKIM. It does not work with PSPWrite and other ZX-81 applications because they are not compiled to do so but Jean is working on a compatible drive to make them work natively."
http://hackedgadgets.com/2008/09/07/psp-phat-and-slim-keyboard/