How can you sell a "Professional Edition License" when the library is released under the GNU GPL? Wouldn't that be disallowed by the GNU GPL?

Systems in Motion can do this dual licensing "trick" with Coin because we are full copyright holders, ie we own every single line of code written for the Coin library.

Therefore we are able to license the Coin library both as Free Software aswell as under a special purpose license for proprietary software vendors. (If you are familiar with TrollTech's "Qt" library, they use the same strategy.)