This is a really useful module. I'm thinking of using this as a solution to separate shipments / preorders.
We sells magazines/books and event tickets. We'd like people who are buying an event ticket to also buy a book.
Event tickets ship a couple of weeks before the show but the book would be dispatched as soon as possible. With the combined shipping module the event ticket shipping is charged as an additional cost to the book solving that problem.
The only issue I have now is that you can only select 1 shipping method for a product with combined shipping. This causes a problem when you have different shipping rates for (in our case) UK and the Rest of the World.
I can change the cck field to be a multiple selection but this doesnt really work, it allows you to select them but it doesnt get honored as I suppose it's never been though of as an option. Any clues on how to implement this, I'm digging in the code now...