I'm testing this out (http://thinkcode.tv/english). I know my current conversion rate, so I'll be able to see if there is a difference (I'll do the math with next week's data, or else random subscriptions from this group will taint it). :)
On the OP example at Huffduffer one clicks a "sign up" button (which has the mouseover feedback you lack incidentally) to get to a page which is for sign up. Hence the affordance is not so important as they're already in the "I need to enter data"-pipeline.
Also FWIW I think messing with default button formats should be avoided unless you're going to make sure that it's clearly a button (affordance again).