I have the 5D and found it immposible to locate a service that would clean the sensor and not invalidate the warranty on the camera. (short of sending the body away for 5 to 8 weeks!)
I was just about to give in and resign myself to renting a body while my own was out for maintence when I decided that I couldn’t be the only one with this issue, so I searched online and found lots of great advice, which when boiled down goes something like this:
Your best bet is to get sensor swabs sized for the full size sensor and some form of cleaning fluid. (I went with the Photographic solutions swabs and their Eclipse brand methanol – which dries much faster than anything water based so you don’t get those scary spots when you are done.)
I’ve been cleaning my own camera now for about 6 months, (a regular issue for me since I shoot on location – which murphy’s law requires to be beaches, windy/dirty cities, and at parties in places with bad air etc.)
After getting past the first bit of reluctance to take a chance with such an expensive component, I’m finding that it isn’t much of an issue at all. I do find that I use more of the swabs than the package recommends…it just doesn’t make sense to me to risk puttin a slightly dirty one back on the sensor in any way, so i only use them for one good swipe.
There are a number of site that can take you step by step through the process, http://www.cleaningdigitalcameras.com/cleaning.html seems to be pretty thorough…