Provided you can get it working on your Ubuntu PC, there's no hardware reason why it shouldn't work.  If it shows up as a routable device, and if you simply plug it in and it acts as a DHCP server, then it is almost certainly possible.

However, there are problems with the current kernel when enabling ip routing and iptables on the chumby.  I haven't investigated much (but probably will soon), but the kernel panics on boot if the option is even enabled.  That'll have to be fixed first.

If it does set itself up as a DHCP server, you might be able to get away with just using it as a bridge.

What sort of USB device does it show up as?  What's the output you get when you plug it in to your chumby, then connect to it via ssh and run:

mount -tusbfs none /proc/bus/usb
cat /proc/bus/usb/devices

Also, when you connect it to your desktop, what IP does it get?  Basically, it may be possible to load the 802.1d and usb ethernet drivers, along with brctl onto /psp, and set the chumby up as a bridge, instead of needing to reflash a whole new kernel.