Sorry if this has been covered already... I tried looking through this thread to find it, but it's so long my eyes started crossing.
Is there a difference between the "HD3870 PC and Mac Edition" and the HD3870 I can find at my local Microcenter, which sells for about $100 less?
I mean, if the hardware's the same...
Yup, there is a difference. the Mac Edition has a special ROM that lets it work with Mac Pros. Apart from that, the hardware is pretty much identical. Unfortunately you'll have to pay that $100 premium if you want it to work in OS X.