Again, depends on who and what. In general, yes there is a separation at alot of companies. But that is not always true....and does not change the fact that programmers with an in depth knowledge of networking are needed in some areas. One such area is developing tools used by networking and network security professionals. There are others as well. I believe it is best to say that in your experience, it is that way. In my experience, there are exceptions and to even promote a division is silly. It isn't incredibly difficult to get an understanding of:

1) Network Operations
2) Data Communications
3) Network Implementation
4) Network Design/architecture (LAN/WAN infrastructure)
5) LAN administration
6) Network Management
7) Network Security

While you will usually not need an in depth knowledge of the configuration of a cisco router, or how to correctly configure a windows 2k dns server (or bind), an understanding of it all from at least the network+ level will help you alot. An understanding of network communications, name resolution, and firewalls can help you figure out why your application (regular app, web app, web service) does not work correctly in your environment. An understanding of system and network security principles can help you set up your development and test environment correctly if you have to do it yourself (which is often the case in my experience). It can also help you when setting up the security for your application for everything from the way it interacts with the firewall to the user authentication methods.

As for design/architecture/implementation....will your application work with a clustered sql server? Web servers behind a bigIP box? Will the web applications talk to an application server? How will all of this react using the technologies chosen, the current code architecture proposed, and the protocols involved?

In a smaller team or one off project the dev may be the person setting up that initial proof of concept environment.

It all depends.