The worm apparently, which came in the form of pornography sent to the group, which had over 50,000 subscribers at the time. The Kama Sutra email, also known as W32/Kapser.A worm, was "designed to overwrite files on infected computers on a specific date."
If you got this email and downloaded the file, it is important that you run antivirus software on your computer. Google promised to try to not do that again.
Postscript From Danny: Google has a post about it here, which gives them a chance to pitch getting free antivirus software through the Google Pack.
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