The Life Hackery weblog lists 50 things you can do with tennis balls, including throwing them in the dryer to speed up and fluff up the laundry:
Throw a few tennis balls into the dryer when you are drying comforters, fluffy coats, pillows, or anything else that could use a good fluffing. [...]Tennis balls can also help any laundry load dry faster—just throw two or three in the dryer and your clothes will be done quicker.
Makes sense that the balls could help the drying process by knocking around your clothes, but further investigation reveals dissent from the laundry expert ranks about this tip.
The Laundry Alternative web site notes:
Some manufacturers recommend putting new tennis balls in the dryer to keep the down from clumping. However, the Soap and Detergent Association does not recommend this practice. The neon dye on the tennis ball could transfer to the down-filled item, or the tennis ball might not be able to withstand the heat. To avoid clumping, it is safer to periodically remove the item and shake vigorously. Adding clean, dry towels to the dryer load can also help.
So do proceed with caution before throwing a ball or two into the dryer.