I've never brewed with rosemary, but please allow me to offer my thoughts - I use it a often in the kitchen, and also as essential oil for aroma in the home.
The aroma of the essential oil when heated combines well with citric fruit aromas, so I imagine it would work well with citrussy hops, Cascades being the obvious choice.
I would recommend using fresh only, since the flavor and aroma are more delicate than when dried. Even so, the flaour and aroma are quite robust - I usually use it in a red wine reduction - about 2 tablespoons of chopped leaves for 4 people, and the aroma and flavor is quite apparent. I doubt you would want more than 4 tablespoons of chopped fresh rosemary leaves in a 5 gallon batch, so the flavor and aroma are subtle.
In the same way we etra the flavor and aromatic oils in hops, I would think the oils in rosmary can be extracted by adding late to the boil or "dry herbing" in the fermentor.
No hard facts here, just conjecture. It would be interesting to hear how your expriment compares to what I write here!