If you have Compressor look in the filters tab in the Inspector window. There's a filter in there called Noise Removal and removing picture noise/grain is what it's designed to do. This filter is present in Compressor 2 and later but I'm assuming it's there in earlier versions as well.
Drag a video clip into the processing window, choose the same codec as your source clip and then add the filter. Do this as a test on a short clip first to see if you like the results. If your final output is for DVD then the noise removal becomes part of your pre-processing set up before MPEG2 encoding. Have a look at this:
http://www.kenstone.net/fcp_homepage/preprocessing_hodgetts.html
- Don
