Can Creme Fraiche be substituted for Heavy Cream in a Cream Sauce?
Actually I would disagree to some extent with the other two answers - I think you can use creme fraiche instead. 

I agree with MStodd that it will certainly give a different flavour and texture to the sauce, but if you're just looking for a healthier base to a pasta sauce, I'd say it's fine to substitute. Also as daniel mentions, you have to be careful not to overheat it (although the same is true of cream), as it will curdle.

In my experience, creme fraiche is great to make a very quick and easy pasta sauce. Put it in right at the end of cooking, once everything else is properly cooked and just heat enough so it is warmed through.

It comes down to whether you are wanting to achieve the same flavours though - if you are, maybe you could try what I would call single cream (not sure what it would be called in the US), and thicken the sauce with a bit of cornflour mixed with cold water.