Re: Transitioning Exchange 2003 to 2007
I went through this transition myself 4-5 months ago. If you search the forum and look to the bottom of this thread, you should find articles related to the transition.
A good starting point is reading this.
The main issues I had was with my antivirus software blocking some of the Public folder replicas being fully moved to Exchange 2007. The rest was making sure I took my time and reading articles like the above. When you move mailboxes, if your clients are using Outlook clients and the old server is still online, they will automatically be pointed at the new mailbox location on the new server.