Spotted on EngadgetHD, who picked it up from TiVoCommunity.com, TiVo has a $200 rebate coming on the Series3 - good from May 27th to June 16th. The rebate form hasn't appeared on TiVo.com yet, but DVRupgrade.com has it up. The rebate form is available as a PDF. Note that it is only good on the Series3.
Amazon.com is currently selling the Series3 for $606.95, so, unless they raise the price this week, that'd bring the Series3 down to $406.95 after rebate! And there are others selling it for around $600 as well. DVRUpgrade sells several expanded Series3 options as well.