Great question — answering as the UphillTech team:
As you mentioned, one of the primary differences between a Presta and Schrader valves are the diameters--with Schrader valves being slightly larger--and therefore the valve holes on bicycle rims are drilled to suit one size or another. Your foremost concern with using a Presta valve in a rim drilled for a Shrader valve shouldn't be 'movement' of the valve stem, as geoffc suggested, but rather a 'hernia' of the tube through the excess space at high pressure, leading to a blowout. Most mountain bike tires sit at a low pressure, therefore you will probably get away with it, however, there is a very inexpensive adapter, sometimes called a 'valve grommet', which is made out of rubber or metal...
Full answer by Dustin Ingram: https://bicycles.stackexchange.com/a/29 (CC BY-SA 3.0).
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