Had this problem, blew some compressed air into the speaker and around the case. All gone, relax people.
All gone where????........deeper into the phones innards of course.......panic people.
Sigh. Never had this problem with my GS2.
edit: added a better picture.
http://i.imgur.com/c6OJN.jpg
You guys are very funny indeed.
There is a simple and real solution to prevent the dust come into FFC.
Just put the 3.5 jack stopper into the jack socket and that's it. No any dust in you phone!
You can buy it on ebay.