If you find that you need to use the ID column to generate incremental invoice numbers and using a formal like =[ID]+1000
There is a problem check out Daisy Anands answer just here
http://community.bamboosolutions.com/blogs/bambooteamblog/archive/2008/04/24/calculated-fields-don-t-leave-home-without-them.aspx
The nub of the problem is that ID's aren't assigned till after the item is created, with the upshot that you'll need a tiny bit of workflow to put the invoice number in retrospectively.
