I just read this post by Ross Smith from the Microsoft Exchange Team in regards to Active Sync when upgrading exchange 2003 or 2007 to 2010. This is a very important article which you will need to take into consideration if you perform an upgrade in the near future. It describes how it works along with the power shell commands you need for the configuration requirements.
It covers upgrades to 2010 from both Exchange 2003 and 2007.
Here are some basic flow diagarms for co-existance for 2003 and 2007 taken from Ross Smith's post.
Please do take time to read the full article before performing the migration assuming your users do use active sync: