It's cheating IMO. As I think Azraele mentioned, it's the interactivity that makes cyber-sex cheating. Looking at porn isn't cheating, because the sexual acts portrayed are already complete, and were not done with you, specifically, as a viewer, in mind. However, typing live, with another person on the other end who's communicating only with you, creates a shared sexual experience. Both parties find sexual enjoyment in their interaction with one another. This experience is something that only two lovers should share. The shared sexual experience makes one feel closer or connected with the other party. An intimate-ness is present, that IMO should only be shared between those who are in love.
Sorry for this rambling, I have a very clear opinion on this, but it's hard to express in a clear way. I'll try and come up with a better way of putting it.