sendmail isn't IPv6 enabled on Linux per default, but it is very easy to enable this support (I'm using Sendmail 8.12.9)
I added this to my devtools/Site/site.config.m4:
APPENDDEF(`confENVDEF', `-DNETINET6')
and rebuilt sendmail. Then I added
DAEMON_OPTIONS(`Port=smtp, Name=MTA, Family=inet6')dnl
to my sendmail.mc and made a new sendmail.cf. After restarting sendmail, it could now receive mail on both IPv6 and IPv4.
I started getting an error from miltrassassin, though:
miltrassassin: connect[120835]: unknown family 54
It turned out, that this was caused by miltrassassin being linked against an older version of libsm.a, which didn't have IPv6 enabled. A rebuild of libsm and a relinking of miltrassassin solved the problem.