I've just purchased a 2006 Z4 3.0si with 6500 miles and it idles roughly when the engine is cold for approximately 15 seconds. I've read several threads regarding a dirty idle control valve. Could this problem be related to a dirty ICV? My dealership says it's due to bad fuel - I don't buy it.

The dealer wasn't lying to you. If they can't find anything else wrong with the car, then it's likely due to gas with too much ethanol in there. Try switching to a different gas station, or better yet try filling up in another area and see if the problem's still there. There are many documented threads about this issue on other bimmer forums as far as I know. I had the same problem on my '04 3.0i when I filled up at Chervon/Shell stations. The problem disappeared when I moved to a different area and started filling up at different Shell/Chervon stations.

same issue here for 15 seconds. I'll try something with less ethanol