I say yes, from this perspective:
A relationship is both mental and physical. Clearly if a person physically gives him/herself to another, we consider it "cheating." However, an equally (some would say more) important element is the mental and emotional. If someone is flirting (other than the lightest and most innocuous of flirting) and writing seductive messages to someone via the internet (or any other medium) then that person is taking mindshare away from the relationship and placing it somewhere else. I have run into very few folks who spend hours flirting and cavorting with someone on the Internet and then turn around and give that same level of attention to their mates. That, in essence, is the crux of cheating--taking resources away from your current relationship and leveraging them in a new one.
And I don't separate the Internet from other face-to-face mediums, because anyone who has met their mate over the Internet will attest that their relationship was as strong or stronger than any face-to-face relationship would have been.
Having said all that, I would hope that I would not out of hand break-up or divorce someone for that reason alone. Cheating is usually a sign of a disconnect somewhere in the relationship from one or both parties. We would need to take a closer look at the relationship and determine why, where and how the disconnects were occurring before I take some drastic action. Maybe I'm an optimist, and I do know there are folks who like to, and will always be, cheaters, but I think that most folks in a healthy, loving, respectful relationship cheat the first time out of ignorance/confusion/frustration/fear. Now, I hope I can remember this foresight and maturity when and if it ever happens to me!
