To see what repository a package was installed via in Ubuntu using apt, use this command:
$ apt-cache policy packagename

Example:

$ apt-cache policy firefox
firefox:
  Installed: 3.0.3+build1+nobinonly-0ubuntu0.8.04.1
  Candidate: 3.0.3+build1+nobinonly-0ubuntu0.8.04.1
  Version table:
 *** 3.0.3+build1+nobinonly-0ubuntu0.8.04.1 0
        500 http://security.ubuntu.com hardy-security/main Packages
        100 /var/lib/dpkg/status
     3.0.2+build6+nobinonly-0ubuntu0.8.04.1 0
        500 http://archive.ubuntu.com hardy-updates/main Packages
     3.0~b5+nobinonly-0ubuntu3 0
        500 http://archive.ubuntu.com hardy/main Packages