IOW, its alright to assume the party has an arcane caster. Its not alright to assume that the party has a 15th level Wizard with Fly and Fireball prepped.
One of the challenges with high level 3.5E play (and this is true of many other systems so it is not, by any means, a dig at 3.5E) is that the range of capabilities that a party could have is broad. An obstacle (say collapsing bridge in Moira) that would be a massive problem with a bunch of fighters becomes trivial if there is a sorcerer with teleport in the party.
I think that there are two options left:
1. Build a realistic module, for opponents of the level of the party, and let the party worry about whether they can handle it
2. Build a model to the capabilities of the party
For an experienced group I would expect 1 to be the better approach. #2 might seem to reduce the challenge and part of the intellectual challenge of the game is figuring out what types of challenges you can face.
If nobody can fly/teleport then you need to think carefully about signing on as caravan guards with a rogue dragon about.
Similarly, if you have characters with knowledge skills, then I'd worry that I'd have to mimic key features of a vampire's powers should we go vampire hunting.