Have a look at this site, My whole pedal assembly was rocking side to side, fixed by adjusting as shown in videos on this site..
http://bicycletutor.com/part/bottom-bracket/
Bob
May 2010
As the cranks enter the bottom braket, there should be two large adjuster nuts. loosen the outer one off and nip up the inner one then thighten up the outer one.
But if the play is that bad I would suggest stiping it all down checking the bearings, re-grease and then put back together
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Try fixfault.com to fix bicycles.