Currently there are no installable apps for the iPhone at all. I am talking retail here. There are plenty of hacks that the wonderful people here and elsewhere have done, but you are being specific about using Terminal Services.
The long and short of it is no. I have used the Winvnc "hack" to access my PC inside my own network, but it's slow. And I am talking about over WiFi.
I have tried accessing through Wifi Outside of my network and it just hangs. I have also tried through EDGE and that is unresponsive as well.
Part of the problem is the amount of RAM available on the iPhone. The other is straight communication between a "mac" and a PC.
I'm sure that in the future, someone will figure out how to allocate some more of the storage space as RAM and also come out with a better version to access through, but there is nothing at the moment.