"...conducted a qualitative study over the course of one year, recruiting subjects from chat rooms such as Yahoo!’s and Microsoft’s Married but Flirting. In all, 76 men and 10 women participated.
Initial online flirtation doesn’t count as cheating, the subjects told Mileham. More than 80 percent felt it was just “talking with a computer.” But online dalliances have a tendency to escalate, Mileham found: 30 percent of those she spoke with—26 people—went on to a face-to-face meeting with someone they met online. And all but two ended up having an old-fashioned affair."
I guess you have to ask yourself (indefinite you) a couple of questions: Wy am I doing it to begin with? Would I feel comfortable telling my significant other about it? I think it would probably count as cheating in some way, shape or form simply because I think I'd be very uncomfortable if it were my significant other doing the virtual flirting.