The current DebianStable distribution was developed under the code name Sarge. It was [http://www.debian.org/News/2005/20050606 released on 2005-06-06 as Debian GNU/Linux 3.1], prior to which it was the DebianTesting distribution.
If you were tracking 'testing' but really meant to be tracking 'sarge', update your /etc/apt/sources.list replacing 'testing' with 'sarge' or 'stable'. If you were tracking stable, [http://www.debian.org/releases/sarge/i386/release-notes/ch-upgrading.en.html use aptitude to dist-upgrade] (change i386 to your arch, or start at http://www.debian.org/releases/sarge/releasenotes .)
Join others in celebrating at a local release party: ReleasePartySarge
New features:
[http://gcc.gnu.org/ GCC] 3.3, and a mechanism for upgrading [http://en.wikipedia.org/wiki/C_Plus_Plus C++] libraries to the new ["ABI"].
See also:
DebianEtch (the code name for the distribution to follow Sarge)
DebianUnstable (code name DebianSid)