brie? there really is no substitute : - )
Grayson is the closest; perhaps Livarot?
Could you use a mix of brie for the creamy and then something nutty like, dang it what's that nutty smooth but not creamy kind of pale yellow cheese ....
Maybe Robiola or a French Pont L'Eveque? Both usually easy to find, if that's why you need a sub for Taleggio.
Taleggio is a "washed rind" cheese, meaning that it is subject to a process that leaves it with a firm, often pungent outer rind, and a soft, more delicate interior. There is a long list of washed rind cheeses here: http://www.artisanalcheese.com/products.asp?dept=1116. Taleggio is also sometimes called stracchino, as it belongs to the stracchini family of cheeses. Perhaps you can find it under this name. I think how you substitute it depends a lot on what it is that you need to do with it. Do you need the same consistency but the flavor can vary? Then a brie will work just fine. If you need the taleggio flavor, then I'd look for stracchino or a washed rind cheese. We recently posted a gnocchi recipe that calls for a taleggio sauce. If taleggio cannot be found, we'd advise just sauteeing a few leaves of fresh sage in olive oil and dressing the gnocchi this way. Good luck!
I would use bel paese or maybe fontina.