I've done it once with with all purpose flour and cornstarch. It was 2 tbsp cornstarch in the bottom of each cup of flower (then just fill the cup with flour after that), and it worked great for me! Because I've only done it once though, I don't know it's always a fail-proof option...maybe one of the more experienced cakers knows for sure?
This link has the conversion and many more. For the cake flour conversion scroll down to Substitutions and Equivalents and read the "Flour" section : http://realfood4realpeople.com/convert.html
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