Go to ConsumerReports.org and sign up to get one of their price quotes. You will get a printout of all dealer costs (including the holdback, which is currently $780 for the 350 RX) and you can simply take this into the dealer and start negotiating from their cost. The holdback should be deducted from the invoice cost to get true dealer cost. If you don't like the negotiating process (who does), just offer them $500 over their cost and walk if they don't give it to you. I'll bet you get a call back.
Lexus sales are down over 21% last month - they are really hurting and the new 2010 is coming out soon. Wait until the end of the month also - you should be able to get one for $500 over cost or better.