Hi Mike
I asked similar questions some months ago and the response I got (I rang cable manufacturers direct) was over time the cable sheathing will deteriorate mainly due to the effects of UV. So was advised to use waterproof trunking/duct. In the end threaded the cable through the wall cavity and so avoided all visability problems.
The info. I picked up along the way about HDMI cable lengths varies depending who you speak to.Often I was told that HDMI is still relatively new technology and the products at either end will influence the end result. But certainly you will get signal loss that increases with cable length so go for a good quality brand. I used a 15m Ixos cable(their premier cable) and the result is fantastic. I was half expecting to have to use a signal booster but for me it wasn't necessary. Bottom line is if you can't borrow to experiement go for good quality and reccommendation.
hope that helps
calvin