MacWhispers claims to have "exclusive" info on upcoming iPod updates.
They report that new 20 and 40GB drives will find there way into iPods (with FW800) as early as this week with the 5gb dropping off the list... but also give a broad time-range (Feb 4th - Feb 11th).
Their information appears to come from these PDFs found on Toshiba's site.
This Spymac thread may be some basis for speculation on impending updates.