I have owned a couple Mercedes, and currently have an '06 E55. I have only taken the car to the MB dealer for oil changes and for warranty work.
Before purchasing my SRT, I spoke with the service advisor at Schumacher Mercedes in Snottsdale, AZ, and was told that according to their dealership agreement with MB, they are not allowed to work on anything but MB.
He was very apologetic, and talked with me about SLKs, and how great he thought the supercharged SLKs were, but he wouldn't even do an oil change for me.
I reallly don't think the dealers would turn away business in this economy if they didn't have to.