Hi BB,
I did a cathedral window pillow recently - here's a link to the thread:
http://www.quiltingboard.com/posts/list/23447.page
I really enjoyed it and plan to also do a cathedral window lap quilt when I get time. I've already bought fabric for it, just need to get busy on it. As to how much fabric you need, if doing the classic cathedral window method the finished block size starts off about 4 times as large. For example, a finished 4" block starts off as an 8.5" x 8.5" square. Yes, it takes a lot of fabric, but remember that this is a completed top and bottom as you go, so there's no need to do a backing on it.
Hope this helps,