Have you seen a gallery showing decades old photographs from 35mm film? All mirrorless cameras do better, much better. Do you think those pictures would get rejected today on the grounds of being to grainy, unsharp or lacking contrast?

Gallery quality has much more to do with with content of photographs than anything else. Light, color, gesture says Jay Maisel. No megapixels, S/N or color-depth there! Sure, it helps to have a better camera but no camera will keep your work out of a gallery if it deserves it.

Depending on your subject, there things to get concerned with when choosing the camera. You did not say about your subjects but if you shoot action, for example, autofocus will be a big issue on mirrorless cameras. Even the Nikon 1 V1 which employs phase-detection and does very well in good light, drops in performance when light levels get low.

For other subjects, the most limiting factor is the choice of lens and there you are rather limited. As you said, 4/3 has the best selection of native lenses. You can also get a NEX with adapter to use Alpha lenses or Nikon F-lenses on the V1 once the adapter becomes available (it was announced with no ship date). The one thing that happens there is that the size advantage drops quickly when you include adapters and bigger-than-necessary lenses. The NEX does have a neat trick up its adapter and that is Phase-Detect autofocus with Alpha lenses. Keep in mind that there is no way to have stabilization with that adapter because the body assumes the lens has it and lenses assume the body has it!