The Panasonic is not a DLSR; there is no mirror. There is also no conventional viewfinder. I'd have bought it in a heartbeat if it were just a bit smaller -- with no mirror and viewfinder a u4/3 body should and could be smaller IMO. That's what I'm waiting for, and if it happens I'm done with DSLR stuff I think.
Consider what you will be doing with the camera. Are you shooting scenery, people, or sports? If you like sports, especially indoor sports, you need something with a high frame rate plus great high ISO performance, and you need a fa$t len$ (the kit lens may let you down in this respect).
Megapixels are not meaningful. The user interface is very important, as are size, and lenses. Consider the body expendable, as it will be obsolete before you know it. The lenses and the system is what you should buy into.
The G10 is the newest and will remain current longer I'd suspect. The Nikon is I think due for revision and you might be able to get a screaming deal on a refurb factory kit (check Adorama or B&H but not on the Sabbath).
Of those three I like the Nikon lenses the most, the Canon body ergonomics the most, and the Panasonic as a concept and what is IMO the way forward. Tough choice. If you don't like something or it's too awkward or too big it'll stay home though, so get what you really feel comfortable with.
Check out dpreview for reviews; they just looked at the G2 Panny and have reviewed the other two also.
and here's their G2 story; the G10 hasn't been reviewed yet but look at the G2 and G1 articles and see what you can read between the lines: