And Google Groups! (Go to Group settings > Advanced, and you'll see how a non-Google Groups mailing list can be archived in a GG
Drawback: you are doing little more than "echo" the listtserv, creating duplicate archives (maybe a good thing); and you aren't fully integrating the other ffucntional offerings (ex: rich text posting, Calendar integration and notifications, etc).
Unless... if the group's email addy was added to the listserv subscription, then the group itself could "send to" the listserv, rather than the listserv merely echoing down to the group as an archive. In other words, more of a "nesting" of the group under the listserv.
But in the end, ain't that a whole lot of circling around your elbow to get to where the toilet paper goes? A group being set to behave as a listserv for certain individuals seems more elegant than a listserv being echoed in (or supervening over) a group. I wanna say it's a kind of "wag the dog" arrangement, probably only offered because of a niche with certain organizations that, for whatever VALID functional reason, can't transition to Web 2.0 paradigm (security, confidentiality, legacy system investment and integration, etc).