What functionality would you lose if you ran Outlook 2003 against an Exchange 2007 server instead of running Outlook 2007?
It looks like you would not have the following (listed in my order of preference):
- Super fast search capabilities (my favorite)
- Scheduled OOF
- External OOF
- Scheduling assistant
- Other features that corporations love like: AutoDiscover, Partial Item download, Managed folders...
Some of the features above are available via OWA 2007 however.