How's life in Redmond treating you?
Exchange 2003 is an over-architected, poorly functional dog of a product, and you need far too many boxes to be anywhere close to scalable. Not to mention the reliance on SQL for the companion products such as OCS, or even SCOM for monitoring.
Clustering and replication is a joke, so forget about HA.
The fact that you must upgrade to 64bit to be supported on 2007 is ridiculous, an absolute gimme for the HW companies with consulting arms, ie, all the major ones.
Exchange2010 is going to be even worse, with seven (I think) server roles that you'll need for anything above 250 users, all on different servers, even if you can put them on a VM. If I didn't know any better, I'd be thinking that the prder had gone out to the Exchange team to expand the number of server roles simply to boost OS sales, bu that wouldn't happen, would it?