The OU clarifies why dairy English Muffins do not violate the rabbinic prohibition against dairy bread (Daf HaKashrus, June '06 - PDF):
Investigation shows that the dairy component in English muffins is in fact batel b’shishim and the muffins are in fact pareve – thereby avoiding the issur to produce dairy bread – but would nonetheless be labeled OU-D based on concerns of bitul issur l’chatchilah and technicalities as to how the OU certifies companies.