A tip for kindle owners (or future kindle owners), make an account on amazon using an american address (just don't buy physical things obivously), then buy gift cards and redeem them on that account. You'll then be considered american.

Most (all?) of the free books are not available outside of the US because Amazon has different wireless deals elsewhere, with different cost structures. In some (most?) places, Amazon would take a loss on the wireless costs for free books, so they replace them with alternate versions which are minimally priced to cover the wireless costs.

I'm in Canada accessing it using my regular IP address, and I managed to get the Advanced Probability and Statistics book transferred to my 3g kindle without a hitch.

I'm in Canada, but visiting the page through a paid US proxy I see them as available.