Now that Service Pack 2 is out there is a lot of conversation around what to upgrade first and, how to upgrade when and if using Clustered Mailbox Servers.

The installation order is, though this is not :

- Hub Transport Server Role

- Client Access Server Role - First upgrade the Internet facing CAS and only after the CAS without internet connection.

- MailBox Server Role - If Clustered, read below

- Edge Transport Server Role

For the Clustered Mailbox Servers, I strongly advise you to follow :

Upgrading Clustered Mailbox Servers to Exchange 2007 SP1 or SP2:
http://technet.microsoft.com/en-us/library/bb676559.aspx

One thing that is worth mentioning is that when the Schema is prepared for Exchange Server 2007 SP2 it is already set for Exchange Server 2010 also.

The article containing What's New in Exchange Server 2007 Sp2 can be found here :

What's New in Exchange Server 2007 SP2:
http://technet.microsoft.com/hi-in/library/ee221150(en-us).aspx 

Pre-requisites for Deploying :

There is only one requirement for installing or upgrading to Exchange Server 2007 to SP2 : Windows Installer 4.5.

I will be posting more in the next days.