It uses your shipping zip code to estimate the sales tax. Since they are obviously using a lookup table to make the estimates from, I would imagine it's going to be pretty accurate for most folks unless it turns out to be "zero" for some reason due to lack of business presence in some state or other loophole.
They're maemo and MeeGo... "Meamo!" sounds like what Zorro would say to catherine zeta jones... after she slaps him for looking at her dirtily...