If you’re facing the issue of visible dust in the viewfinder of your Canon 5D (not to be mistaken with dust on the CMOS sensor), read this superbly detailed cleaning procedure (complete with pictures) which can eliminate or reduce the amount of that dust — also don’t forget to read the minor correction to the procedure.
Read this to get an understanding of the technicalities behind the source of the dust problem, which interestingly does not occur in other Canon Digital SLR cameras such as the 1DSmkII, 20D and 30D; the occurrence seems to be unique to the 5D.