If you're downloading over the wireless from the B&N site have you tried running it through a proxy? I find http://codeen.cs.princeton.edu/
is a really good site for it.
But if you're running it through the B&N store using wireless you're out of luck I think. Unless you were able to run your wireless through a dedicated computer that ran all internet traffic through a VPN/proxy...
I get my Nook in about 5 days (live in Australia) so I'll be working on this problem soon. I might see how I can go about running the wireless with an American IP within a rooted Nook.