For the most part no- but if you somehow get a huge gash across the screen and then it suddenly doesn't work, you're not going to get warranty coverage.
Usually the warranty is voided if there is visible user damage that could affect the performance of the computer. Usually this could be a dent or crack, but scratches on the casing of the computer, most likely not. You wouldn't believe how many people pretend that their computer or ipod was never dropped, but the Genius bar can tell. I like the spec cases at www.speckproducts.com. The y make good protectors that do not compromise the form factor too much.