Appears to be an issue with the Checkout Field Manager plugin.
When I un-check "Allow editing of core required checkout fields"
things work again in IE9, but now my reason for the plugin is
useless : (
Ok. I jumped the gun on exposing a culprit. I had done two
things to my checkout page when it started working again. First, I
used an alert shortcode at the top of my checkout page that comes
w/ my Sommerce theme to let customers know of an issue with IE9.
Second, I disabled the Checkout Field Manger setting. I thought it
was the plugin, but it turns out that it's something funky with the
page that adding a shortcode on miraculously fixed.