One of players' most frequently asked questions since BC emerged has been when -- or if -- Blizzard plans to make flying mounts usable in classic Azeroth. It's an attractive proposition due to the sheer size of Kalimdor and the Eastern Kingdoms, and how convenient it would be to take a fast flyer to places that aren't well-served by flight paths. CM's and developers have said that the programming and time required to "retrofit" the old world to be usable by flying mounts isn't worth it, and that they'd rather spend developmental resources on new content.
A player who got irritated with all the begging -- and someone who I can only assume has access to GM powers on a private server -- decided to take an aerial tour of several zones in Azeroth in December 2007 to document the territory Blizzard would have to redesign in order to make flying mounts feasible. It's a really interesting look at how Blizzard exploited perspective and line-of-sight in order to make buildings and zones appear bigger and more built-up than they actually are, and there are a few places in the video that I haven't seen even on websites dedicated to the "secret places" of Azeroth. I'm not sure why this video hasn't popped up previously but it doesn't seem to be tagged or categorized in any way, so that might be why. Check it out; flying in Azeroth would be great to see these things, but there's no denying Blizz would have a lot of work to do.