I recently found a new product that some of you might be interested in. In stead of using felt paper there is a product that is "wall shield" made by Vapro Shield. There site has a lot of products pertaining to rain shields as well as the wall shield product. I am sure it is probably not as cheap as felt paper but for those who want to go the extra mile it might be worth checking into. I think there claim to fame is in the Contemporary building field where a portion is left exposed between a slatted exterior design.
http://vaproshield.com/products/wallshield/rainscreendetails/