This was exactly the problem; I was able to partition and format from within Ubuntu 8.04.1 once I specified the device correctly.
The only subtlety was that I had to umount the device in order to format the partitions - I do not believe that step is shown here:
http://code.google.com/p/beagleboard/wiki/LinuxBootDiskFormat
Both DOS and Linux partitions showed up on ubuntu when the SD/MMC card was unplugged and replugged.
Thanks, Tamas and everyone for your suggestions!
Best Regards,
John