Gmail is part of Google Apps. That's pretty mainstream. Personally, where I work, anyone using any version of IE is screwed. We don't test for IE. I haven't checked to make things worked in IE as a regular part of development since 2009 and I have worked at a pretty big company that is often talked about on HN.
A few years back I was working on an app with some moderately fancy CSS and JS. We desperately wanted to drop IE6 support but after we pulled the revenue numbers per browser, we worked out our IE6 users could pay for a junior-mid range developer to just sit there and fix IE bugs all day.
I wonder what that's doing to the maintainability of your code base. I'm sure the overall cost is higher than just the pay of a single developer ruining your architecture. Supporting IE6 is more than just about fixing bugs.
Also, something to possibly consider: reach out to customers and ask them how much they need IE6 support. Unless it's healthcare or government, I'm not sure why anyone would use IE6 by choice. Would deprecating support for IE6 lose every single one of those customers?
A big chunk of the old IE users were military people deployed in the Middle East. I'm of the understanding that they have/had some sort of Internet Cafe setup on base, and I guess they were running older browsers.
And what happens when the amount of your users drops slightly or that hired dev is now worth more money? And what about the other costs on complexity in your codebase? I think the revenue needs to far eclipse one dev's salary to justify keeping IE6 support.
