There is the FlashMobWiki?. http://flashmob.wiki.taoriver.net/
The first GFM (Global FlashMob) took place on oct 25th 2003. See http://flashmob.wiki.taoriver.net/moin.cgi/That_20was_20GFM_20_231
GFM #2 is sceduled for june 19th 2004. http://flashmob.wiki.taoriver.net/moin.cgi/List_20GFM_20_232 Add your city please.