Wow - this is interesting. Engadget is reporting a kick start code to enable eSATA on the Series3! It looks like it was found by user Spike2k5 on the TiVo Community forum. Reportedly, this only works on unmodified Series3 boxes - so if you already installed a larger internal drive, this won't work.
NOTE: This is obviously not officially supported. TiVo could disable this at any time, and you'd lose recordings when the second drive stopped working. It could also be buggy, etc. So use at your own risk. Still, very cool - and it looks like they have it ready to go officially, just waiting for Cable Labs to wake up.