How strongly do you rate Image Stabilization?
Image stabilisation is a 'nice' rather than a necessary feature as far as I am concerned. It can be useful, but it many of the situations where I might need it I also want to stop blur from subject movement, so a faster shuuter speed is necessary. Good image quality at higher ISO values is of higher value to me than image stabilisation. That said I'd rather shoot at base ISO an m43 cameras, and to achieve that when I don't need a fast shutter speed a light tripod with a large ball head yields sharper results than IS.
It seems all Oly bodies have IS built in, whilst Panny offers IS in some of their zoom lens, none of their pancake/primes.
I've not felt any great need for IS on the 20mm, its not that long and its fast. If I need to stop down I don't find it hard to hand hold, but then I've got that tripod if I really want ultimate sharpness.
Would lack of IS be a deal breaker for you?
No - a minor advantage on shorter lenses. More of a consideration on longer lenses, but not a deal breaker at all.
I'm coming from a P&S and worried about not having IS.
Will have the G3 + 14-42mm but looking possibly towards G3 + 20mm = no IS.
Stop worrying. You really don't need IS on the 20mm, if you need to stop down such that your shutter speed drops below 1/30, you can try one of these.
Optimise your hand holding technique. It is possible to get accepatble results at slower shutter speeds.
Use some form of improvised support - wall, tree, rock whatever.
Use Flash (if appropriate)
Use your stabilised kit lens.
Use a light tripod. You don't need a behemoth, the G3 + 20mm is a small light combo and there is no mirror slap.
Otherwise, shoot at f1.7 and auto ISO (limited to the highest ISO you are happy with the results from)