For a few hours now, http.debian.net is also reachable over IPv6. The service is still experimental, as the code makes a few assumptions that are not yet automatically verified and the IPv6 GeoIP database is sort-of experimental too.
Note that the results between a request over IPv4 and one over IPv6 may vary quite a bit. Requests over IPv6 are redirected only to hosts that support it. A great number of mirrors, however, don't support IPv6.