Being from the northeast I have made quite a few Maple beers. If using syrup in a porter I found that 1/2 gallon of Grade B (expensive early season syrup) with half added pre-boil and the other half added at flame out gave a subtle maple flavor. I think the trick is to make a porter on the mild side so the roasted grains are balanced by the syrup.
IMO the best option, if you can swing it, would be to use Maple Sap as your brewing liquor. It gives a nice woody, maple flavor. This is how I did my last batch.
That's a great idea about the sap, I guess my next question would be how much to add to a 5 gallon batch to get a decent maple flavor?
About 7 gallons I would guess. I think that saints was saying he used it for the entirety of the brewing liqour. It is a subtle flavour when it hasn't been boiled down.