No problems with my 3IS but, then again, I have a different engine to the OP.OP reported this problem back in October - I wonder if the problem has been resolved? It sounds weird that turning on A/C would increase the engine rpm at idle ...?
I wonder if timetorelax has experienced any idle issues on his 3IS 250 (none I'm aware of)?
turning on a/c does increase rpm on nearly all cars
A/C compressors run directly off the engine through another belt