This info on the back reference project page is no longer true. The content api in D6 does the work (and similar the field api in D7). It respects the fixed-number done in the field, all kind of references are true.

Along the same lines is Corresponding Node References, which seems to work similarly to BackReference. The main difference between the two seems to be with handling single-value or fixed-number value fields, which CNR does not seem to handle well. As always, when choosing a module for your project, look over the existing options and choose the one that matches what your requirements need.