Congrats on your upcoming RX400h purchase. We just got one ourselves in July, and absolutely love it.
Before you go through a local dealer, make sure you do a pretty good Internet search for other large metro areas near you.
I live in Hawaii and there was only one used RX400h for sale on the island when we decided we were ready to purchase and it didn't have a nav system (which is a requirement for my wife). Knowing I was able to get it shipped, I started hunting in SoCal for inventory that met my qualifications. 2008 with low miles. (hard to do, but they are out there) We found at Mercedes Benz of Valencia (LA area) that had 28k miles on it and they would give us a break on the price equivalent to the price of shipping. After having a friend go look at it to make sure it was in decent shape, test drive it and make sure there were no obvious flaws, we pulled the trigger.
Not having to sit in the negotiating room with the salesman running back and forth to the 'manager' was worth it's weight in gold. I literally sat at my desk while working and exchanged text messages with the sales rep until we decided on a price.
We ended up getting a 2008 400h with 28k miles on it in July of 2011 for $30,050. We did pay $865 to ship to Hawaii, so total we paid $30, 915. They even delivered it to the port for us, and I'd imagine most dealers are familiar with the shipping process for over-the-road shipping.
So don't get locked into your local inventory if it isn't exactly what you want. You can probably find exactly what you are looking for online and have it shipped to you. Internet sales are usually a little more agressive in their pricing too because they know if you are buying on the internet you are much savvier and less prone to accept 'an offer you can't refuse' in a dealership.
I will never again set foot inside a dealers 'sales' office after the ease with which we were able to do it over the Internet. We did use USAA's car buying service to find it, so if your bank has some sort of service like that, tap into it- most will also give some sort of a guarantee as well.
Good luck with your purchase!