A GTK/Sugar hackfest (a bit smaller and more focused than the Paris SugarCamp) will be held in Prague at the brmlab, from October 28-30.  The hackfest agenda is under development on the sugarlabs wiki.
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Does this happen annually?
Good question – something related to Gnome hacking, if not necessarily GTK, would be a good annual hackfest, since it’s a major part of the OLPC builds and distributions.