The Treo Pro was announced without carrier support for the USA, meaning that the only way to get it is through Palm at a steep $549 plus tax and shipping. However, wmpoweruser.com has a great article detailing the way to get the Treo Pro for nearly free.
Since AT&T and Apple offers a steep discount, but a very tightly controlled process for acquiring an iPhone 3G, the best thing to do is get an iPhone 3G for around $199 and sell it on E-Bay, which goes for around $700, saving the SIM.
Order your Treo Pro and pay about $50 in difference.
This is a circuitous route to take, but if you really want the Treo Pro, it sure takes the zinger out of the price tag.