I have tried several bags for my m4/3 system. I have a Think Tank Retrospective 5, which is a nice little bag -- emphasis on "little." It easily carries my GH2 with the 20 attached, along with a 14 and a 45 stacked and either a 90mm Tamron or a GF1 as a second body. Plus batteries, filters, etc.
Domke makes various inserts, one of which fits inside my small Timbuk2 courier bag, and holds pretty much the same gear. Neither of these will hold a very large kit, so I'm still on the hunt for something else.
Can't have too many bags