Here's a cool site to check out - New Cover is a blog showcasing re-designed book covers.  Once a book is read, a new design is created and posted.  There are some cool ones, I kind of like the set that was done for Jonathan Strange & Mr. Norrell (a book I would like to read someday).

Are there any other books you've envisioned with different covers?  Overall I don't usually think of redesigning books, but I have a say a good cover can make all the difference in getting me to actually look at a book.