Given the recent network-wide conversion to timeline, cover photos have become more and more of a playground for designers.
Recently, while Rett was creating cover photos for our MS150 fundraiser, he discovered a clever cover photo exploit: if you upload a tall cover photo, it will get cropped when you view your profile on the website, but not when you view it from the Facebook iOS app. If you ask Facebook, the official cover photo dimentions are 851px x 315px - but we found that a cover photo as tall as 697px would still be fully viewable in the iOS app.
This means you can have special messages (or "mobile only content") for your mobile friendly friends. All you have to do is upload a cover photo that is 851px x 697px and you'll be all set. Facebook will crop the image for the website view, but show the whole thing on mobile!
The caveat? The Facebook iOS app only gives the cover photo layout to real people -- brand pages still have the old layout -- so until that gets updated this is only really useful for your own personal Facebook account.