Outlook 2003 with Exchange 2003/2010 coexistance
Alot companies who held off from upgrading to Exchange 2007 are upgrading to Exchnage 2010. Most companies also have not moved on from Office 2003 and although they are looking to deploy Windows 7 with Office 2007/2010 in the near future. If you like one of my customers are planning on using Outlook 2003 with Exchange 2010 need to be aware of the changes you must make to your Outlook profiles.
This MS article provides the information you will require to change on Outlook clients that will be connecting to an Exchange 2010 mailbox.
One thing which recently caught me out was coexistance with users still on Exchange 2003 and when they tried to access Calendars or other Mailbox folders for mailboxes migrated to Exchange. In this customers environment they were only enabling the encrpytion when the users mailbox was migrated. Therefore users who had not be migrated received an error when opening calendars of users who had been migrated.
Therefore I recommend that you modify the encryption settings prior to moving any mailboxes to Exchange 2010 so Outlook 2003 clients are not affected by this problem.