I've attached several USB keyboards to my n900, didn't face any problem. Of course in order to use USB keyboard you must have h-e-n installed. In h-e-n UI follow normal procedure for connecting usb flash drive for example, just skip mount step. Have in mind that keyboard is most probably "Low Speed" or "Full Speed" USB. After that open extkbd UI, your keyboard should be listed there. If it is not, please get dmesg output and post it here. BTW you need to setup keyboard layout only once.