I am late on mentioning this, but a writer friend of mine, Stephen Parrish, is now running an armchair treasure hunt in support of his upcoming first novel, The Tavernier Stones. I have not read the book yet, but I've enjoyed the excerpt he posted quite a bit, and am really looking to getting my hands on it when it comes out in May.
If you're into ARGs, which are becoming all the rage these days, I think that something like this sort of armchair treasure hunt might be right up your alley. Armchair treasure hunts have been around for a while, and if this one is any indication they are as tricky as heck. As well they should be -- the clever winner gets a 1 carat diamond (no joke) as a reward. So be sure to stop by Stephen's site and check that out.