What About “Maybe”?
blog.coordinatr.com
blog.coordinatr.com
This is how to properly communicate contentious design decisions to your users. For that reason alone I'm voting it up.
Another interesting take could be to add an optional "If|Unless..." clause to the Attending and Not Attending buttons, for people to clarify their intentions.
In that regard, we've taken a page from 37signals' book "Getting Real":
"The best software has a vision. The best software takes sides. When someone uses software, they're not just looking for features, they're looking for an approach. They're looking for a vision. Decide what your vision is and run with it."
Hope that kind of explains where we're coming from and why we haven't given creator's the choice of response types.