A moon orbiting a gas giant could harbor life.

IMHO, if Io was orbiting Jupiter at Earth's comfort zone, it 'could' have more of an earth-like atmosphere. That goes for Titan and Saturn. Although, Titan is tidally locked with Saturn and as Dan pointed out, that presents a problem on a moon('s interior core and atmosphere, radiation, temps, magnetic field...) A star or sun plays an important role. SDAI-TECH is right when he mentioned the importance of planet formation and elements needed/required during formation. Elements(heavy elements stayed behind), speed and gravity play a vital role in a star system's early history. The Jovian planets played a large role in determining amounts and where those vital elements would be located. Maybe Io would be another Earth-like planet in our system if it wasn't trapped by Jupiter. Or it could have been more closer to Venus or a bit more like Mercury. Lately, I've become a firm believer in Panspernia. I believe that Earth's abundant atmosphere and ingredients for life were ignited or enhanced this way. As Dan also said, having a moon like ours definitely helps.

That's my (very brief) explanation, in laymen's terms. Mark, I am a sponge when it comes to all this lately. It's been years(coughcough) since I took astronomy and I have to admit that my reading comprehension levels are much higher than those of my writing skills. :) Reading Alex's, Nick's, Marcy et al's papers and keeping up with new discoveries has certainly helped.

Marcia