There seems to be a lot of confusion about flying in Patch 4.0.1 and in Cataclysm. Here's an article about the Master Riding skill and how it works. This hit with Patch 4.0.1. It costs 5000 gold but there are slight reputation discounts. Basicly this allows your mount to move faster.
http://wow.joystiq.com/2010/10/16/patch-4-0-1-its-still-a-long-strange-trip/
Looks like when Cataclysm hits, cold weather flying will cost 250 gold.
http://wow.joystiq.com/2010/09/09/cataclysm-beta-old-weather-flying-will-cost-250g/
Once Cataclysm hits, if you want to be able to fly mounts in Azaroth, you will have to buy a Flight Master's License for 250 gold from a riding trainer, but you don't have to have the Master Riding Skill mentioned above. You will probably have to be Level 60 and have bought the Cataclysm expansion.
http://www.wowhead.com/guide=cataclysm&faq
I've heard complaints about opposing forces being able to attack from above and there are rumors Blizzard will do things to modify this but not word on that yet.
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