I did my first ganache under fondant cake last weekend. Used semisweet chocolate and white ganache. Came out fine. Just be sure to clean up around the bottom of the cake before applying fondant, so you don't have chocolate in the fondant you cut away. Learned that from Sugarshack's video! 
Susan
It very much depends on how thick fondant you are using - if it's thick enough the dark ganache will not shine through. But if you roll it too thin you might get a problem with the dark colour coming through.
Some people use a layer of marzipan on the ganache and then fondant on top of that.
I roll my fondant thin and never had a problem with chocolate ganache showing thru, even thru white fondant
I use semi-sweet ganache under white fondant all the time. Good luck!
I roll mine to 2 mm or approx 1/12 of an inch but if I have something dark to cover I might go up to 1/8 of an inch
Do you guys know if you can use milk chocolate chips to make ganache or does it have to be semi sweet? Also, do white chocolate melts work for that or no? Thanks!
Caralovescake, you can use milk chocolate chips or white chocolate chips (the ratio with white chocolate is different though, it is 3 to 1)
What this thread was so helpful, I too have been wondering the same thing.
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