mercurial
The general mailing list. Busy, but has a high signal-to-noise ratio:
Mailing list archives, searchable on Gmane, MarkMail, marc.info and nabble.com
The apparent etiquette on this list is to include thread participants in replies to list messages, or rather, to not trim participants from replies to the list.
mercurial-devel
The developer mailing list. Created in October 2006:
Mailing list archives, searchable on Gmane, MarkMail and marc.info
mercurial-packaging
This is a low-bandwidth list is for people who build distribution packages of Mercurial to coordinate releases and also discuss packaging-specific issues.
Created in Feb 2010, see also Matt's announcement: http://selenic.com/pipermail/mercurial-devel/2010-February/018655.html
Subscribe here: http://selenic.com/mailman/listinfo/mercurial-packaging