You will have to implement v-usb on the Arduino to connect directly.
http://www.obdev.at/products/vusb/index.html
Some projects emulating keyboards here:
http://www.obdev.at/products/vusb/prjhid.html
Or if you have an old computer you could emulate a PS2 keyboard:
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=34901&highlight=keyboard+ps2
PS2 might be easier and can be used with a PS2 to USB adapter for modern PC's