According to this article from the BBC and this post on Microsoft's Exploring IE Blog, Microsoft has planned to automatically upgrade Windows XP, Windows Vista, and Windows 7's IE to the latest version.
Microsoft has stated that they will also allow people to opt-out of this update through the use of the The Internet Explorer 8 and Internet Explorer 9 Automatic Update Blocker toolkits and will respect any previous declined automatic updates. I assume managed corporate IE managers can also block whatever they want.
Microsoft's blog post states that the release will begin in Australia and Brazil to be "scaled up as time goes on."
I'm wondering, from the perspective of a web designer, how rapidly these upgrades are likely to be propagated to average end users, but also especially those in tightly-regulated corporate IT infrastructures. Given best (or common ;) practices in the corporate world, as well as average end-user behaviour, how soon can I stop investing a significant amount of time in backwards-compatibility with IE6 and 7?