Hi,
If you don't fancy the idea of breaking open an USB keyboard, you can always use a keyboard module like this one:
http://www.ultimarc.com/ipac1.html
http://www.ultimarc.com/ipacve.html (economy USB model)
Basically it is a bunch of switches and it shouldn't be a problem to interface it with a Arduino.
A more complicated option (but very powerful) is to build a Processing sketch with the Java Robot class. Robot is a class to control mouse and keyboard events:
The arduino can talk to Processing which sends key presses to the host.
/Camille