I don't think anybody is working on a app to connect a 360 controller. The people working on usb host mode (now called h-e-n) have not finished that app yet and made it work stable and they only made it work on a few N900's (if you want to know more about it search this site: usb host mode). You will need them to finish first before you can connect any usb dongle/memory stick etc. to the phone and make it see it. Then you need to find out if its possible to make it work with a dongle outside of the Xbox 360 and then get somebody to make a driver for it and then som kind of app/gui for the N900.
I think the chances for us to see a 360 controller work on N900 is slim to nothing and i would surgest you aswell to get a Ps3 controller