OpenStreetMap
The leisure=stadium tag implies building=yes/stadium/multi_storey/.... Otherwise, the thing is a pitch.
Don't believe me? The Memorial Coliseum and the Rose Garden Arena are both stadiums. As of 12-mar-2010, someone has decided that the Rose Garden Arena also needs an explicit building= tag. See the difference yourself.
http://www.openstreetmap.org/?lat=45.531731&lon=-122.667919&zoom=18&layers=B000FTF
Interestingly, there are some facilities that need the building= tag to render. For example, a bus station renders but a train station won't. Compare Union Station to Grayhound. (Well, or you could but the railway=station tag has been removed from Union Station, sigh.)
http://www.openstreetmap.org/?lat=45.528022&lon=-122.676062&zoom=18&layers=B000FTF
Discussion
Comment from Wynndale on 13 March 2010 at 08:42
How would you tag Rosslyn Park rugby round then? I tagged it as a stadium because there is fixed accommodation for spectators, but it is only on one side of the pitch and I mapped the whole area of the ground.
Comment from marscot on 13 March 2010 at 20:17
I dont know there area but I would tag the buildings as building inside the leisure stadium