I very boringly tend to start all these comparison questions with: try before you buy.
You're only asking about IQ, but there may be other issues to consider as well. In addition to IQ concerns, you've got interface concerns, you've got lens compatibility concerns and maybe other concerns. They all matter, you have to figure out how and why for your needs.
IQ: M8 should generally outshoot the G1/GF1, better dynamic range, cleaner hi ISO files, maybe not by a lot, though.
If you put any stock in DXO Mark rankings, G1 is lower than M8, NEX 5 is better than M9 (yeah, M-nine).
The GH1 is better than the G1, the GH2 looks like it will be better than the GH1.
Crop: M8 is 1.3x, NEX is 1.5x, G1 is 2x. G1 loses most of your wides, helps most with 50mm+ lenses.
Lenses: M8 does best with 35mm & wider lenses in the corners at wide apertures. G1 will smear corners more than NEX, but NEX also seems to have RF-legacy wideangle corner issues.
If you have C/V 12-15-21-25 lenses, like to shoot wide-open and like sharp corners, the G1 will generally disappoint, the NEX might be marginally better.
Interface: M8 and G1/NEX are night/day, totally different, even if you try using OVFs on the G1/NEX. NEX interface seems much improved for enthusiasts with the new firmware. I've always liked Pany's interface, but the GF1/G10/G2 do finder/screen zoom for manual focus easier than the G1.
NEX has no EVF, G1 has an excellent onboard EVF. I really need the EVF, some do fine without.
Will you love live view or only tolerate it?
Other Stuff: NEX has video, G1 doesn't.
Can anyone here make even a subjective comparison between the G1 and the Leica M8 image quality? I have an M8, but I'd like to pick up a G1 for an extra digital. I have a pile of CV lenses that I can put with it.