I decided to go USB since an IR or RF battery powered unit wouldn't work in the winter for me (the battery would freeze).
This keyboard is cheap! $15 plus shipping
It is extremely small and looks high quality too
The one downside is that the keyboard is *so* small that it is not practical for typing large amounts of text. However, in a carpc environment, it is ideal since typing is usually limited to typing URLs or small commands etc.
Overall, I highly recommend this for a simple carpc keyboard solution!