The Wembley Stadium bowl volume is 1,139,100m^3. A smartie is .0000026001m^3, so you need:
438,098,534,672 smarties
Supplement from 04/13/2007 11:08am:
That probably assumes zero air gaps - dunno what the result would be then, but I'd guess it would reduce the number by somewhere between 1-10% (likely to be around 2-5% reduction)