To add to this I think one of the advantages being mentioned for GWT as opposed a JavaScript library is the fact that GWT is java so you can apply unit testing etc. and treat it as regular java, whereas JavaScript is difficult to test and troubleshoot, because of its underlying design features.
On the other hand I think something like jQuery would come in very handy if you had to add some things to an already existing application where you can pick and choose what you want and where. It is more light weight to implement and less binding. Think about Java and C++

, I sure do not want to keep track of garbage collection having used Java for so long

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Actually my statement above maybe incorrect, watching one of the presentations on youtube I remember the google engs saying you can edit your JavaScript as well so it really is being placed as the JavaScript choice for Java developers.
Plus the other thing with pure JavaScript is that you still see the nuts and bolts and can, if you so desire (being a Linux fan I hold that dear), make modifications and implement things differently.
It all comes down to what do you consider to be your core area and how much control you need over what is being put in a page you produce. With one or the other you get a huge amount of your work done by someone else. I mean the things you can do with jQuery are great and GWT is nice too, but getting JavaScript to work with so many browsers and quirks can be difficult and that is why I value

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One thing that is kind of a concern for me is that with this web 2.0 craze applications are again moving to the client and that has its benefits and disadvantages as well. Yes, you do put all the code in on the server but what happens on the client is not under your control, so as someone supporting and writing this stuff I can see some how some problems may crop up that are hard to troubleshoot. The security aspects of this stuff are also interesting.
My 0.02 cents.
Regards,
George