WHen implementing in a situation like yours (going from Ex2k3 to Ex2k7 or Ex2k10) you have to go through the deployment guide and re-implement all of the security reccomendations as newer versions of exchange (Ex2k7 or later) and AD hosted on W2k8 or later have different requirements.
You'll need to look into re-applying security (via powershell) on your mailbox servers & cas servers for mailbox and calendaring web services. You'll have to look into creating a MAPI throttling policy for your BES service accounts, if you have AD running on W2k8 or later you'll probabally have to look into configuring NSPI throttling. You can't just add an Ex2k10 server beside an Ex2k3 server and expect things to work
Luckily this is all in Rim's deployment and administration documentation. Make sure all of the prerequisites have been met and you'll be fine.