The Rio Central does this to an extent. When you encode a CD on it, you have the option of making two different versions of the same track. You can encode at 320kbps for use on the Central, and also have a 96kbps version at the same time for use when uploading to a portable device. I do believe the bit rates are user defineable.

This is pretty convenient, but I have been looking into making a server with enough disk space to allow me to store my collection in FLAC. Then, whenever I want, I could encode into whatever compression format I want. That way, I would only have to rip my CDs once, then encode to whatever however many times I want without having to screw around with my CDs ever again.