Here's an interesting tidbit. Apparently, CDW's Microsoft Licensing sales reps have been told that if your back end is running Exchange 2003 Enterprise, that your front end must as well. Some Microsoft sales reps are telling the same story as well.
Well, I got word from KC Lemson (through my brother) that this is not true. That it's perfectly legal to run Standard on the front end and Enterprise on the back end. Apparently, this wasn't the case with 2000 as front end/back end functionality wasn't available in Standard back then.
So save yourself $2200 or whatever it is when setting up your Exchange organization in this manner and get Standard for your front ends, unless you need Enterprise to do clustering or for other features only supported on Enterprise.