Did you happen to notice that bolded section above? That's the key line. It's one thing for Google to halt support for features used mostly by its business customers. Businesses should know enough to have their employees on the most recent and secure browsers.
We plan to begin phasing out support of these older browsers on the Google Docs suite and the Google Sites editor on March 1, 2010. After that point, certain functionality within these applications may have higher latency and may not work correctly in these older browsers. Later in 2010, we will start to phase out support for these browsers for Google Mail and Google Calendar.
Google Apps will continue to support Internet Explorer 7.0 and above, Firefox 3.0 and above, Google Chrome 4.0 and above, and Safari 3.0 and above.
Gmail and Google Calendar, however, are used by millions of regular people for their regular lives. Perhaps the vast majority of those people are smart enough to be using newer browsers, but there's no doubt a handful aren't. By taking away support for a vital service -- yes, email is vital to most people -- it will likely have the intended affect of forcing people to upgrade their browsers. Let's hope it works.
Google didn't specify when it will drop support for Gmail and Calendar in IE6.