Most answers given here are false. It is perfectly legal to have
an underscore in a domain name. Let me quote the standard, RFC
2181, 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.
See also the original DNS specification, RFC 1034, section 3.5
"Preferred name syntax" but read it carefully.
Domains with underscores are very common in the wild. Check _jabber._tcp.gmail.com or _sip._udp.apnic.net.
Other RFC mentioned here deal with different things. The original
question was for domain names. If the question is for host
names (or for URLs, which include a host name), then this is
different, the relevant standard is RFC 1123, section 2.1 "Host
Names and Numbers" which limits host names to
letters-digits-hyphen.
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