When you have multiple Form Content Types with each their respective form templates assigned to the same Form Library by some magic it will automatically save a new form in the correct Content Type. How it does this ?
The new form’s XML contains an InfoPath processing instruction with a “href” attribute set to the XSN template URL. At save time the Content Types configured in the Form Library are iterated and a match is made based on Template URL. If no match is found it will use the default Content Type.
Even works in code if you do a SPFileCollection.Add() so no need to specify the Content Type explicitly during saves !
It doesn’t seem to take Alternate Access Mappings into account so that’s a bit of a downer, but still powerful.