If you like to know which package versions are included into some
particular Debian/Ubuntu/Backports release, rmadison tool from
devscripts package could be the answer. For example:
$ rmadison -u debian,ubuntu,bpo mercurial | cut -d "|" -f 1-3
mercurial | 0.9.1-1+etch1 | etch-m68k
mercurial | 0.9.1-1+etch1 | oldstable
mercurial | 1.0.1-5.1 | stable
mercurial | 1.5.1-2 | testing
mercurial | 1.5.2-1 | unstable
mercurial | 0.7-8 | dapper/universe
mercurial | 0.9.5-3 | hardy/universe
mercurial | 1.0.1-5.1~hardy1 | hardy-backports/universe
mercurial | 1.1.2-2ubuntu1 | jaunty/universe
mercurial | 1.3.1-1 | karmic/universe
mercurial | 1.4.3-1 | lucid/universe
mercurial | 1.5.2-1 | maverick/universe
mercurial | 1.0.1-5.1~bpo40+1 | etch-backports
mercurial | 1.3.1-1~bpo50+2 | lenny-backports
It will show "official" information, regardless of repositories you
have configured on your system.
Please note, that versions between releases are not kept as a part of
official repositories. However, you can give a shot to recently
launched http://snapshot.debian.org service, which allows to access to
old packages based on dates and version numbers uploaded to Debian
archive since around 2005.
Similar tool you may be interested in is whohas, which goes far
beyond Debian family. It's able to show versions of available software
for other distributions (Arch, openSUSE, Gentoo, FreeBSD and even more
- 14 distros at the moment).
Hope that helps.