I've had the same problem of light corrosion on one of the brake pistons with my 2006...
I didn't feel like taking it apart completely at the time, so I only took the caliper off the front fork with the hose still attached, took the brake pads out, and pumped the pistons out just as far as I thought to be possible without the bottom of the pistons passing the seals (and causing the system to empty itself all over the place
)...
With the pistons extended as far as I dared to go, I polished the pistons by wrapping a shoestring around them and pulling it back and forth...(= the only way to get around the pistons without disassembling them completely)
This really did the trick, the pistons were smooth again, apart from a slight discolouration where the rust had been, did the same trick again with a clean shoestring soaked in silicone grease - and this caliper has been fine for two years since...