The people behind ExtJS are funny. First they have changed their license to the LGPL, without understanding it in any way, now they've changed it to the GPL without understanding it in any way. They claim that server side code which creates HTML pages which contain ExtJS must be GPL, wuahahaha.
I've been running LGPL and GPL open source projects for several years, had lots and lots and lots of discussions about the very fine points of the LGPL and GPL and let me tell you the ExtJS guys have absolutely no clue. Which is a shame because ExtJS was a nice library. Now they're history. No sane company will touch a project which changes it's license frequently and has such an attitude towards the - dangerous - GPL.
Update: My latest thought. Although ExtJS is GPLv3 and the developers claim (falsely) that your backend needs to be GPL too, for internal/intranet applications you still can use ExtJS as you're not distributing ExtJS.
Update 2: A little madness has a nice analyis on the switch.