Do you know that in Exchange 2007; there are 5 different roles, MAILBOX, EDGE TRASNPORT, HUB TRASPOIRT, and CLIENT ACCESS SERVER AND UNIFIED MESSAGING.
If you are a typical organization with only one server then you might be thinking that you will need to install all these 5 roles but this is not true. There is a way around for this type of typical scenario. For a single domain and single AD you only need 3 roles i.e. Hub Transport, Mailbox and CAS. This is must go for a typical server setup but you will lose the additional security feature of Edge Transport which provides anti-spam as the main feature. For Unified messing you will need IP Telephony in place. So it depends how you plan.
If sometime later you want to use Edge transport it can be done because Edge transport is not supposed to be a part of AD. It will be deployed at Edge and will not be a part of the AD but all the email in and out will be from Edge Transport. So that time you will need to make some modifications.
I hope now you must have got the idea .. J