I think different people have different ideas about these things, making an "ideal" setup hard to quantify. For example, I prefer to keep gateway- and infrastructure-type services on physical machines. I don't see any benefit to virtualizing small business network infrastructure and some benefit to not having everything on one server (though having multiple facilities influences me there). Virtualizing the desktop, mail servers, web servers--the stuff I think of as user-facing network services--makes sense to me, just as keeping the backbone of the network on physical servers makes sense to me. But of course, that's just how I think.
The only real practical limitation, it seems to me, is the server hardware. What can the server effectively virtualize? At least some of the rest of the deployment decisions are about administration philosophy and preference.
Just for what it's worth.