For those of you who are consulting this will now likely be a common topic.
Should a client wait for E2007 or go with E2003 now and then move up later.
I have put together a small slide deck which can be found in the downloads section which details some of the options that should be considered. Feel free to customise it as you see fit.
To download see here:
Also for more info on Exchange 2007 and who is using it, take a look here at the MS case study page:
I hope this helps a few people.
If you have suggestions or slides of your own feel free to get in touch and I will happily add them to the site.