I think the main issue here is not IE6 but all the modern browsers (FF 3.5, Safari 4, IE8) that do not support this feature. Targeting Chrome users only is not that appealing.

The feature was added to webkit so both safari, chrome and other will get the feature and so is firefox https://bugzilla.mozilla.org/show_bug.cgi?id=472529

This is HTML5, they will support it soon if not yet.

That's why you would use it where available, instead of targeting.

if (browser.supportsWebSocket) useWebSocket(); else useComet();

While the numbers are falling, IE 6 users still account for a sizable percentage of the people that buy things from my site. Ignoring them would be "doing something wrong". Guess it just depends on what you're doing.

Also in case you are doing something that spreads virally, not supporting IE6 can change the viral loop from increasing to decreasing (you could lose 5% of invites/feed messages/emails/whatever depending on app).

It really depends.

I recently did a bit of consulting work for a guy who has a large number of domains receiving primarily organic search traffic. Around 20% of the people hitting those (several million a month) are still using IE6.