I third the photo idea. Photos will mean the most to your parents. Make sure you design some small signs for you and your DH to hold during your wedding photography. One that says "Thank" and the other that says "You." It's a perfect way to end the collection of photos you give them.
I really like the digital photo frame idea too. There are supposed to be some killer deals on these on black friday. I imagine there will be great deals throughout the holiday season. I also looked into designing and ordering a photobook online until we decided on the following...
We're paying a bundle to have our photographer make one of her OUTSTANDING albums. Well, she offered a deal on miniature versions for our parents. They won't be bound in leather like ours, but they will be 6"x8", bound like a book with a faded full colored image as the hardcover, and perfect gifts for our parents. We actually need three of them since DF's parents are divorced. They are a little more expensive than doing the photobooks online and they are a little smaller, but the pages are thicker, the design will be unique, and I know they are going to be fantastic. I'm almost more excited to surprise them with these than I am to get our album after the wedding. Since we won't get them until 6 months after the wedding, in the meantime we are planning on choosing an engagement photo and framing it nicely for each set of parents. We will give it to them the day before the wedding.