I'm taking Ac3d will be built in to the terrain editor? I imagine that would be the way the licensing fee would work.
Anyway, I believe the new TE will use .X objects, which can be transferred in from any program - just not Ac3d... -
If not, Ac3d can import most files built in other programs, so should be a non-issue.
Again - would be nice to find out the details.