What functionality would you lose if you ran Outlook 2003 against an Exchange 2007 server instead of running Outlook 2007?http://www.microsoft.com/exchange/evaluation/featurecomparison.mspxIt looks like you would not have the following (listed in my order of preference):
Some of the features above are available via OWA 2007 however.
What about running Outlook 2007 e-mail client with Exchange Server 2003?
Most of the advantages of using Outlook 2007 with Exchange 2007 don't apply if you are using it with an Exchange 2003 server. For instance, the Scheduling of OOF and internal vs. external OOF. Those rely on services that have been added to Exchange 2007.
However, if you are using Outlook 2007 in cached mode and are still on an Exchange 2003 server the search will be much faster.
Apple iPhone for now ignoring synch to Exchange, other e-mail platforms Configuring, validating and monitoring
One Point to mention: If you use OL2003 with E2007, Managed Folders are available and working. The only thing that doesn't "work" is that users won't see the comments of a managed folder...