Every six months or so, Valve (the company that publishes Half-Life and CounterStrike, and runs Steam, one of the largest game content distribution systems) runs a survey of the hardware used by the vast majority of their customers. The results in this survey are somewhat slanted since most respondents are gamers, but the last survey showed 2% of over a million respondents were using a resolution of 800x600. Adjusting for the audience, this would indicate at least 5% of the general internet population is using this resolution. Many web site analytics applications show similar numbers around the web. The resolution is dying, but it is still in use enough to not be ignored.
By the way, the valve survey results can be accessed at
Valve Survey Summary