I have been thinking about this for a long time, and today I felt the time was ripe. I have set the official LICENSE of the OctoMY™ project to GPLv3 with an EXCEPTION. You can see the new license here.
So what is the EXCEPTION? Well, it basically grants me as the original copyright owner the full right to change the license for the main-line official distribution at any time, without the terms of GPL coming back and biting me in the but.
This EXCEPTION also states clearly that any non-official non-mainline (i.e. copies/redistributions etc). are free under the terms of the GPLv3 forever without the exception.
I think this is reasonable. I want to own OctoMY™ (after all I spent all my bacon on this project), while at the same time I want it to be free and I invite people to make copies and spin off projects etc.