Separate physical network for backup?

This is a general question rather than a product-specific one, so I'm not really sure where to put it. I'd like to ask the question of the Avamar, Networker, and Data Domain communities.

To give you some context, at present we have two physically separate (i.e. separate cables to separate NICs on our servers) networks - one for production data and the other for backup data. We are looking at consolidating everything into just one physical network, but we'd like to see whether others are doing the same. If we can point to a whole lot of companies and other educational institutions that don't use a physically separate network (I don't mean separate VLANs) for backup then it will give our request a lot more weight in the eyes of management.

So, to those of you who are responsible for such things, do you use physically separate networks for production and backup data?

Thanks.