My current feeling is that it's not worth the bother and you'd be better installing full Chrome and hiding IE unless you also need the compatibility for internal apps.
Chrome Frame fit the bill nicely. You include a tag in your HTTP header to request Chrome Frame. If it hasn't been installed then IE8 renders the page. The nice thing is the site self-selects that it wants Chrome Frame so your legacy sites continue to get IE8. Even better, your users are still launching IE so you don't have the support hassle of this site requires this browser while this other site requires this other browser.
Meanwhile this gives us an opportunity to modernize our legacy web applications. Chrome Frame has been a great solution for us.
We have limited resources, therefore we stay focused on our area of expertise and stick to standards (zero flash/java/silverlight to minimise support area and odds of yet another thing possibly going wrong), develop edge (HTML5, CSS3, responsive...) first, and have fallbacks for less compliant browsers (e.g IE9), and possibly block access to part or entirety of our products to old browsers, while clearly informing the user as to why he should live with it or get on board, and that he can use Chrome Frame to not change anything to his habits but raise the "features" bar (most of the resistance comes from ivory tower project managers and BOFH sysadmins, while users mostly don't care or agree). This is, for us, the only viable solution in the long term.