I noticed this when I start my 98 900 when it's cold; when it is not in gear, the rpm's drop slightly when I release the clutch. I never noticed this before. But it's -10 here in Vermont, and I've been prestarting in the morning.
Earthworm's probably right. When you let the clutch out, gears in the transmission have to spin up in the cold oil, putting a load on the engine and causing the RPMs to drop. Once it all warms up and the oil thins out, it shouldn't have as much of an effect.