Ideally, IE8 should be your minimum. There's a large contingent of web users still on Windows XP and IE9+ is not available for it. XP is in extended support until 2014-08-14 & yes, I'm counting down the days.

I, for one, would never use this while it remains incompatible with a large segment of the web-browsing public.

So, yes, I think it would be a big deal. IE8 would be even better. But with IE9 compatibility, at least it would be feasible to start using this when IE8 fades away.

I'm not sure IE9 is such a huge win. This already supports IE10, and every Windows 7 computer will be auto-updated to IE10 (if Microsoft holds to it's decision to automatically update from now on).

XP users will never get IE9 or IE10, and IE8 can't be supported unfortunately. So I guess Chrome Frame will be necessary to target IE8 users.

IE9 support lands today, and we've decided against supporting earlier versions of IE - too much headache and code bloat/hackishness to justify addressing the ever shrinking < IE9 user base.