I would say they are for different things really. ExtJS is all about pre build widgets you can compose into other widgets, saving work, but sticking within how they work and look (skinnable). jQuery gives you much more control but at a lower level (widget wise).
One of the advantages of Growl being run at the OS level is that all apps can fire to Growl and the user see it. An in-browser Growl is going to be limited to firing events only while the user is on the same page.
i made a mockup of the same concept w/ an ajax heartbeat triggering the messages (mootools 1.11 instead of jquery), but never released it. kinda shakin' my head now :)
tried on the iPhone. Works, but the messages don't appear near where the user action is being done so if I am not zoomed out, nothing appears to be happening.