I went to Kosovo 4 times in 2011, got two stamps 3 times, once they forgot one. Serbians are not so good with stamping, I didn't get one when leaving to Bulgaria. That means that you can just ask the Kosovars not to stamp your passport when you leave Kosovo once you entered it from Serbia.
What is important to remember is that entering Serbia from Kosovo after entering the latter from a 3rd country may prove impossible or problematic. I read an account of a Polish cyclist who was refused entry to Serbia although some German cyclists in front of him were allowed to enter. Afterwards, he was smuggled in by Polish peacekeeping soldiers through a green border into Serbia :) I met those same soldiers one day in Macedonia when they were on a weekend off and they confirmed this story.
So... even when you are refused entry to Serbia after entering Kosovo from a third country, you still stand a chance if you meet Polish KFOR! ;-)
