In principle, the answer seems to be that as far as the products
themselves go, SharePoint and Exchange can co-exist. If you are using
network load balancing for the CAS role and for SharePoint, it may be
advisable to assign different IP addresses to the NLB system so that you
can fine-tune the NLB settings for each product.

however, seems to be ForeFront. We are currently struggling
to install ForeFront for Exchange on the dual-purpose system and we've
now found a blog entry that states that ForeFront for SharePoint will
not install on a system that has Exchange on it.

So it looks like we've got to split out our installation, unless MS
change the way ForeFront installs and works.

The only reason for not having SharePoint 2007 on the same
server as Exchange is if you need to use email enabled libraries. The reason
for this is that SharePoint uses the inbuilt SMTP of IIS and cannot use
Exchange for this. When you install Exchange it overrides the normal SMTP
process within IIS. Have to say have not tested this deployment with
ForeFront for SharePoint, very weird. Hope this helps.

I believe that is no longer an issue - pretty sure WSS3/MOSS can read
Exchange2007 export format (this was supposed to be added in the
MOSS/WSS betas, but I haven't confirmed it actually made it in). You'd
need to setup a connector/rule on Exchange to export the mail intended
for MOSS to the file system, then configure MOSS to look at that
location instead of the SMTP dropbox location.