How come there isn't a single camera with an UNDO option in the menues for when you delete something you shouldn't have??
Firstly, if you don't know how filesystems work, imagine a book with a table of contents, 'deleting' a file on a CF just deletes the entry in the Contents page, the photo is still available in the book, the camera just doesn't know where it is. Once you take another photo, it can overwrite the photo, or part of it.
Compare to Win/Mac "recycle bin", when you 'delete' something, it doesn't delete the page or the table of contents, it changes the table of contents to say the photo is sitting in the 'recycle bin' (until you empty the bin, then it's the same as above).
So, when you delete something accidentally and want to 'undo' that delete, the camera would have to have a sort of 'recycle bin', and your card wouldn't empty for new shots until you empty that too. Personally, I never delete in-camera unless it's a really ballsed up shot, pure black, pure white, or i'm out of space. And if i delete something when i'm out of space (and then empty the recycle bin, if there were one, to free up the space), then take a new shot, it's probably overwritten that photo anyway so there can be no 'undoing'.
Basically, just don't delete in-camera unless you really really are out of space. And if you're regularly running out of space, sorry, but just buy more cards, $50 is a case of beer or another 8gb of space, your choice.
If you really really really mess up and delete something accidentally, there's another option, don't shoot any more on that card, just swap to an empty card, and use a 'file recovery tool' (something that can find the pages of photos without the table of contents) on your computer when you get home. But then, if you've got a spare card, why are you deleting in camera?