There is also — technically — a microbial difference between the American and French versions of crème fraîche, as my friend and regular commentator Nick R. points out. One can purchase such a culture, which is composed of a mix of Continental bacteria, and gives a more authentic product. If you’re interested you can buy it here.
Also, the missus, who never misses a chance to pinch a penny until it bleeds, writes in to comment:
If you’re going to buy heavy cream AND buttermilk, why not just spring for the creme fraiche?
For the record, I wasn’t necessarily suggesting anyone go out and buy both of them. I just meant that a little of the sour stuff could be added to some cream in a pinch (though for the record I’ll bet buying both is still cheaper than actual crème fraîche, and you’ll have leftovers to make buttermilk pancakes and whipped cream to boot!).
Crikey! Who knew crème fraîche could be so controversial!