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In the "Settings" tab, after adding a new Update Center (or enabling a disabled Update Center), the GUI gets blocked for some time. I think this is due to the download and checking of the catalog from the Update Center being executed in the AWT Thread. It should be done in background, or a popup with a progress bar should appear. Tested on Mac OS X, I believe it reproduces on all platforms
It gets improved since this issue has been filed. Now it is fine at least for disabling Update Center providers.
Right, I think it's fixed already in NB6.5