Google vs. Microsoft may not be zero sum, but...
Will Google Apps hurt Microsoft? Yes, and probably without even trying.
...Today I saw something that should petrify Microsoft. I was visiting a large financial services company and couldn't help but notice Google Apps running throughout the floor as we strolled past. It turns out that the company is actively rolling out Google Apps because it figures the vast majority of its users have almost no need for full Microsoft Office functionality.
This was long the premise behind why enterprises would switch to OpenOffice, but I suspect that OpenOffice wasn't disruptive enough. It's not purely a question of license cost: It's also a question of training and deployment costs.
With Google Apps, the end-users are largely driving the change themselves. They want the change, because they're already using Google Apps (Gmail, Docs, Calendar) at home. Having these at the office is a welcome surprise, rather than an unpleasant surprise foisted on them by the IT department.
So, while the competition between Google and Microsoft might not be zero sum, Microsoft has a lot more to lose from the duel than Google does, which has seen Microsoft barely ripple the surface of its advertising hegemony despite a concerted effort to create waves. Google hasn't hurt Microsoft, either, but then, it hasn't really tried.
Yet.