I've read a few articles on sizing Exchange 2007 roles but they all talk about how much RAM, Cores, HDD you need per role. This is fine for Enterprise but I deal more with Small to Medium sized business.
What is the limit that you would suggest splitting from a single Exchange 2007 server and spreading the roles out? I've done quite a few 20-50 user deployments with 2007 and a single server works fine. I have a few clients in the 250 user range that want to deploy Exchange 2007. The users are medium to low load users for the most part.
If a single server wouldn't cut it would you split it into 2 servers for CAS/Hub on one and anoter for a Mailbox server? Edge is not needed as I have a spam filter appliance and UM isn't needed.
Real world examples of successful deployments would be nice too, just for proof of concept.