I got a challenge on the 8th and tried to accept it now (I've been away).
When I clicked the e-mail I got a page saying that the game doesn't support multiple browser windows (I already had the game window open, but idle).
So, I closed both windows and clicked the e-mail again, and it said the challenge had expired.
Did it expire due to the elapsed time (the e-mail doesn't mention any expiration) or because I had already tried to accept the challenge?