Update: Well, I gave 3D Brush a try using VMware Fusion and Windows XP on my iMac (Intel Core Duo) and it's a "no-go". I was able to install it, but I think the big issue is RAM. 3D Brush needs at least 700 MB of RAM and if I assign that much to the virtual machine, my iMac slows to a complete stop. If I assign less to the VM, 3D Brush launches but pretty much slows to a complete stop also. My poor iMac only has 1 GB of RAM, so I think you'd need at least 2 GB to even start to do something small in 3D Brush, and even then I'm not sure how the performance would be. This is the first time I've run an app in Fusion that launched but performed this poorly. Usually if the app launches it runs pretty well or it doesn't launch at all, or it launches with limited functionality. 3D Brush pretty much locked up Fusion and my Mac.
I think the only realistic fix is a Mac version. Oh, well, I gave it a try...