That RFC has been superceded by http://www.ietf.org/rfc/rfc2181.txt – From section 11, “Name syntax”:
“The DNS itself places only one restriction on the particular labels that can be used to identify resource records. That one restriction relates to the length of the label and the full name. […] Implementations of the DNS protocols must not place any restrictions on the labels that can be used. In particular, DNS servers must not refuse to serve a zone because it contains labels that might not be acceptable to some DNS client programs.”
(this was raised on SO a while ago as http://stackoverflow.com/questions/2180465/can-someone-have-a-subdomain-with-an-underscore-in-it)