I cant find a relevant rfc or something similiar, but it seems to be quite common to me...
see: * https://www.cryptosys.net/pki/manpki/pki_distnames.html * http://www.modssl.org/docs/2.4/ssl_reference.html (search SSL_CLIENT_S_DN_UID)
Or to ask from the other side... where would you put your username inside of the certificate? In our case the CN contains the full name of the user...