I change lenses on my G1 as often as I need and don't have any indication of dust on the sensor. The Panasonic and Olympus 'SSWF' is dare I say it, a bit better than that used by some manufacturers, and so long as you momentarily switch the camera off during lens changes you shouldn't have a problem. But I'm not one to 'never say never' but look on the bright side and the lack of a mirror will make a wet clean of the sensor even easier.
You are right to think the G1 kit lenses are good, they are very good, and only get a 'kit' designation because of the speed. Like somebody else on this forum, I have found myself using them far more than I thought over my M lenses because I like the kit lenses so much. But the really wonderful thing about the G1 is that it makes changing between legacy and dedicated lenses so easy, no menu changes, just be on aperture priority and turn the dial from AFS to MF.
Steve