Edit: Scratch that, per Melissa569, there is an eBay seller who ships to the US quite a bit cheaper, ~US$28 now including shipping (see comment thread). At any rate, looks like there are a small number of options if you want to buy the UK Mooncup in the US, at least for now. (Another eBay seller claims they cannot ship to the US; hopefully other sources won't get policed soon too.)
I recently got this email back from Henna at kuukuppi.com (aka leastore.com). They are in Finland, and I originally was on their site looking at the Lunette. But then I realized they also carry the Mooncup (UK) (which mooncup.co.uk isn't allowed to ship to the USA thanks to Keeper), and I was dying to know if she could ship them to the US? The answer is YES! Her reply to me is below.
Thank you for contacting us! We haven't got any other instructions for not delivering Mooncup(UK) to the United States. The shipping costs for menstrual cups are equal worldwide 1,99 €/cup. If you choose more of them or other products you can check the applicable shipping costs with the shipping estimator tool in your shopping cart.
kuukuppi.com customer service
Anyway, they sell the Mooncup (UK) for 28.90 EUR, so with shipping, 30.89 EUR. ($40.88 right now per xe.com.) Granted, this is over $9 more than if mooncup.co.uk could ship directly to the US, but if you don't happen to have friends who would forward it to you (plus it would cost something for them to forward it anyway), this might be a great option.
My hunch is that other online stores (especially outside of the UK) might be able to do the same, but I haven't actually looked into this.
Anyway, go wild! Buy some MCUKs!!! Wear 'em as party hats!!! ;-)