My g/f's dad had the dealer install one of those black colored aluminum sliding covers and he loves it. I thought it was really neat. They make them motorized, but I think I'd prefer the manual one like he has, faster and easier. It looks great closed and open, and can be locked in any partially open place if you wanted to leave some of the bed open to carry a tall item and the rest covered. They are pricey, and usually you buy the rails and the cover seperately from what I've seen in the various automotive catalogs. Plus, they are black... but often times you have a black bedliner or roll in liner anyway right?
This is really similar to the peragon link posted, but there are more sections, which seems like it would make it roll up smaller.
And it can be installed in any truck bed from what I've heard and has water channeling to run off to the back (like a car trunk).