OtterBox has been making cases to protect cool smartphones and other devices for years. The cases come in all sorts of varieties and offer different levels of protection from the simple silicone case to more complex cases that are water resistant.
OtterBox has launched a new case for the Palm Pre that addresses an issue I had not really thought about for slider phones -- how to get a case that lets the phone still slide. The new case is called the Tandem Series and it is made in two pieces that clip onto both halves of the Pre and allow for protection against drops without restricting the phone.
The case covers the volume and power buttons with a thin silicone layer and all other ports and buttons are accessible with the case on. Pricing and availability for the case are unknown. I would bet the cost would be in the $20-$30 range.