I think it would be a mistake to drop the 17", unless they drop the price on the 20". But maybe the 20" and 24" are selling better than I imagine?
Or unless they are finally going to give us a decent headless iMac... get mini prices back down to the $500/700 range, drop the 17" iMac, have a mid-tower Mac at $999-1299, start the 20" at $1399, the 24" at $1899, and the Mac Pro starts at about $2099 up to $3000 (when you drop it to the lower CPU config). Seems like a pretty good line up.