Supposedly, the way around the restrictions is to use a VPN (HotSpot Shield, AlwaysVPN, etc.) to hide your actual IP and give you a virtual one that is located in US. Thus, B&N will think you are located in US. You can then, apparently, use your standard credit card to download ebooks.
For me, I got some mixed results. I haven't actually bought a book from B&N, but I did get several from their freebies collection. Most of them worked, but for some reason, there are still one or two that won't download. The 'purchase' goes without a hitch, and the books are available to download in my library, but when I try to download, it claims my credit card is invalid/expired. Don't know how to get around this.
But as AnemicOak said, there are plenty of other sites you can get ebooks from. In addition to the ones he listed, you can also go to ebooks.com, and the new Borders ebook site seems decent, too. You can also frequently get them from WHSmith's ebook store, but since the prices are in Pound Sterling, they might be a bit steep for some books.
Happily, though, I can confirm that Larsson's books are quite fairly priced. They (along with several more) are currently available for a discounted price (35% off). I bought The Girl with a Dragon Tattoo a couple of days ago, and it's GBP5.19 (about US$7.9).