I've never Twittered--it's about as appealing as Morris dancing--and this fine post on it at The Age of Uncertainty pretty much nails my feelings (with the addition that I have not owned, and never will own, a mobile phone). But, having said that, reading said post and then wandering off through the net via some of the replies, I came across this lovely cover.
It's for Iain Sinclair's new one, Hackney, That Rose-Red Empire, and it's a dustjacket that folds out, and out, into a great big map. Now, I may be a hopeless nerd, but I think that's rather cool. I just wish I didn't find Iain Sinclair to be such a pain in the hole, because then I could justify buying it.
UPDATE: In the comments below, John Self of Asylum mentioned another book with a great fold-out dustjacket, the Picador first edition of Mick Jackson's thoroughly enjoyable The Underground Man. John was kind enough to share these images of said jacket in its various states of unfoldingment.