It's fixed already

I am very impressed. Apple has already closed the security hole (see below) in their Software Update mechanism.
With this surprisingly fast fix Apple proves how seriously they take security issues (responses to the latest SSH/Apache vulnerabilities have already been very timely too).

Get your Update here and don't forget to verify the checksum (2c039c683b7001defc35f93ba1f68db3e33e41fc) of the update after downloading AND dropping it onto Stuffit expander:

/usr/bin/openssl sha1 /Path/To/SecurityUpdate7-12-02.dmg

From now on all Apple Software Updates will be cryptographically signed and the new Software Update client 1.4.6 checks for a valid signature before installing new packages.

And there's more: this update does contain a COMMAND-LINE version of Software Update which comes very handy when administering several machines via SSH. See man softwareupdate for more Info.