Let's start with the more competitive AmE-to-BrE category. Here we've had some nice suggestions, and the hono(u)r nearly went to primary in the sense of 'preliminary election'. Reader-in-Ireland mollymooly had suggested this at the end of 2009, noting that the Conservative party had an open primary to choose a candidate for the House of Commons seat for Totnes. Perhaps it should have beaten staycation last year--but it came to my attention a little too late. But it was ousted as frontrunner in the last day of nominations, when SP nominated a gerund that has both been discussed in the news this year and made its way into UK news. And that gerund, The 2010 American-to-British Word of the Year is:
Like, for instance, his [Cameron's] current 'shellacking' (love that word) over a supposed lack of vision and confidence in the recent Guildhall speech. [Skol303 comment on Nick Robinson's blog]
Vince Cable being torn a new one by Kirsty Wark on Newsnight...she got him so rattled he developed a Herbert Lom-like twitch (left eye) halfway though the shellacking by Wark (I kid you not). [samandmai comment on digital spy]
So thank you, SP, for a fantastic nomination!
And on to the BrE-to-AmE winner. This is always a tougher category--in part, because I live in the UK, but mostly because of the lesser impact that UK news and popular culture makes in the US. The winner is not a particularly 2010 word--instead, it's one that's been making steady progress in AmE over the past decade. But in hono(u)r of the near-culmination of the Harry Potter film adaptations, the British-to-American Word of the Year is:
...in particular, the adjectival use to describe hair colo(u)r and, to some extent, the noun use to mean 'a red-haired person'. Twice this year I've heard from US parents (including Mark Allen) who have said that their children use ginger in this more British way because of the influence of the Harry Potter stories, which features the red-headed Weasley family, including Harry's sidekick Ron. (Here's my old post on the topic.) The much-discussed new Google n-gram tool shows 'ginger hair' steadily increasing in American English books since 1995, though Harry Potter was not released in the States till September 1998. In British English books, however, there's an increase in the Harry Potter days (after some years of decline), but what looks to be a decrease as we come toward(s) the present. It's hard to say if that's meaningful--and unfortunately I don't have access to any British corpus that takes us up to date. In the more reliable Corpus of Historical American English, there are 8 uses between 1940 and 1979, none in the 1980s, five in the 1990s and 8 in the 2000s, which seems to show the Harry Potter effect. It's harder for me to find incursions of the noun ginger in the meaning 'red-head' in AmE, since one must search for word strings, not meanings. All I can think to do is to note that the Urban Dictionary entry for the noun ginger include some contributions that spell color without a u. Further evidence is welcome in the comments.
Also welcome in the comments are your thoughts on whether I've done an effective or abominable job in choosing this year's Words of the Year. But if you don't like them and didn't nominate any, I reserve the right to roll my eyes at you. Through the computer. Ouch.