I think this sounds like a lot of fun. My main issue with it is that the last regular game I played in, a 4E Forgotten Realms campaign ran by my favorite GM, Wirty, basically had the same plot.
I know it seems like a game like this might get monotonous. Dragons and dragon cultist do sound like they could get old, but there's quite a bit of room for variety. Each of the dragon type has a different personality, environment, and element.
I know some of the blogs I follow have discussed using Tiamat before. The two that come to mind first are Tim at The Other Side and Zak at Playing D&D With Porn Stars. Both have made some interesting posts about Tiamat and/or her cult, so you could probably get some good ideas there.
Even if I don't run it, I'm interested in what Wizards does with this. There's a lot of potential.