I will concur with Julius. I have run Presta tubes in Schraeder drilled rims without a grommet for years with no problems. Having said that, I always make sure that ALL valve holes are deburred before I use them, presta or schraeder. All it takes is one little piece of metal, grommet or not, to puncture a tube.
Jay Sexton Sebastopol, CA
You can get schrader rim converters from sjscycles.co.uk
http://www.sjscycles.co.uk/
Personally I've never bothered. Good tubes should be strengthened around the valve (so pressure hasn't been an issue) and if the rim hole is cut badly with sharp edges I've lined it neatly with electrical tape and not had a problem.
Cheerio
Julius Naim
London, UK