I have a similar setup to Andrew with the logitech harmony ultimate that came with the smart hub, plus touch remote and a second Logitech smart remote (no screen).
I have the smart remote downstairs to control volume and skip tracks when im pottering around the house. (because I have my music streaming around the house with synchronized multiroom audio using an Airplay server configured as a zone in MC - but thats a whole nother story! see my airplay instructions in my thread if you want to try this out your self
http://yabb.jriver.com/interact/index.php?topic=95760.0 )
I can control theatre view from down stairs too so this setup would work for you but I don't use it that way. (just to avoid confusion you dont need to do any of the airplay stuff I metioned above, I just mention it so you know my use case and that the logitech remote works well from another room to control MC.)
I also have a windows MCE compatible IR receiver from a Hauppauge TV remote kit plugged into the main HTPC via USB and and IR blaster plugged into the Logitech Hub.
So I can control MC on the HTPC from anywhere in the house with either of the Logitech remotes.
I havent thought about it in detail before, but I think this is how my remote configuration works;
Logitech Harmony Touch Remote or Logitech Smart Remote < > Logitech Unity Wireless RF < > Logitech Smart Hub > IR Blaster > MCE IR Reciever > HTPC USB port > MC21 set for MCE remote
The Logitech smart hub uses a proprietary wireless RF transmitter and receiver in the hub and the remotes to work pretty much anywhere in the house.
The secret is having an MCE compatible IR receiver plugged into the PC and the RF capable Smart Hub with smart remote or touch remote.
If your Smart Hub has line of sight with PC MCE IR reciever you wont need the blaster plugged into the Smart Hub.
If your going to get an MCE IR receiver, make sure its a proper MCE one and not just a receiver that emulates HID USB keyboard commands as the proper MCE ones work better with MC21. (the keyboard emulation ones dont always seem to work reliably)