Respond.js is mandatory if you want to use mediaqueries in IE8. This is the polyfill that enables media queries for the browser.
Modernizr only offer test for mediaqueries. According to the doc, in older non-supporting browser the test will always return false.
Another alternative is adapt.js by Nathan Smith creator of the 960gs grid system.
Update:
Modernizr is now adding support for html5 tags in older version of IE as stated in the doc
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