In some builds of Magento (Community and Enterprise), there is an issue with IE9 and all of the credit card fields being disabled. This is on step 4 of the one page checkout, and users will not be able to enter any information, or even use the dropdown select box to choose which type of credit card they are going to be using.
There are fixes out there that justify this fix by setting the Meta Content-Type to force IE8 (or an .htaccess fix that does the same), which is fine, but forcing a browser to downgrade isn’t really ideal. There is a cross-browser solution to this that doesn’t change the meta-type, luckily!
Browse to your /skin/frontend/..template folder (if you’ve got one, if not, browse to the /skin/frontend/default/default/js/ folder). Open up the opcheckout.js file. Around lines 641 AND 647, (inside the switchMethod function), replace :
var elements = form.select('input', 'select', 'textarea');
var elements = form.select('input').concat(form.select('select'), form.select('textarea'));
These 2 lines should fix your IE9 issues, as well as be compatible and work with Chrome/Firefox, earlier builds of IE, and others.