"[...] wrap it up and sell it as their own. [...] With no mention of us at all."
Nope, that would be illegal in most countries. You cannot deprive the author of his ownership and tell people the work was yours.
If someone would repackage ExtJS he would be required to keep the headers and original copyrights intact. So by definition he can't "sell it as his own". This just shows how uninformed the ExtJS core developers are.
"Since we started we have been open source and that is the route we will continue on."
Running a company with a distorted view on the GPL ("backend code must be GPL if it generates HTML with ExtJS in it") is not "beiing open source". The open source argument is just a fake front to make more money.
IANAL of course.
The sad thing is though that Java open source projects have learned over the last 10 years that an Apache license leads to much more success. Apache will lead to companies using your library, more developers and in the end more users. More users lead to more support contracts and more license sales. Few users lead to few sales. And with lots and lots and lots of users someone big will buy you (Oracle, Sun, Google, Yahoo, MS). The GPL model only works for standalone products like MySQL, not for libraries. I've needed to learn this the hard way too.
The most important point is: Open Source success is build on the trust of your community. Lots of developers are let down by ExtJS. If you destroy the trust - as ExtJS did - your project is history. Go jQuery UI, go!
It will be interesting how they fare against e.g. Tibco GI which is commercial but liberally BSD licensed.
Thanks for listening.