The NSLU2 is a good low-cost companion server for the Neo 1973 (the initial hardware platform for OpenMoko). It has one 10/100 ethernet port, two USB 2.0 ports, and easily supports connectivity with the Neo via Bluetooth (using a low-cost USB Bluetooth dongle).
I am creating a MokoSlug firmware distribution for the NSLU2 for this purpose. It will be based on the Angstrom distribution, which is built using the OpenEmbedded build system (the same build system which is used for the OpenMoko software).
Like the SlugOS firmware distribution for the NSLU2, it will initially run from internal flash but will allow you to boot from an external USB disk or flash key if you need more storage for installation of additional software packages.