Yeah, I got the same problem. But I think it's a graphics card problem, not Mountain Lion. It works on my Macbook Pro Retina with Nvidia card, but not on my iMac with Radeon. It didn't on 10.7 either, now they are both running on 10.8.1. Latest version of 3D-Coat (3D-Coat-V3-7-18C).
And yes, it is quite important to fix that bug. Use my Wacom with the iMac.
Tried registering on Mantis now, but it must be very slow. Have not gotten any e-mail back yet.