I was told (unverified) that Microsoft dropped the UDP-based new mail notification feature in Exchange 2010, so that Outlook 2003 online clients do not receive real-time updates about server-side mailbox changes anymore. This appears to affect any kind of mailbox operation, not just new mail arriving, but also move and delete operations etc.
Cached mode works differently, so that would explain why it is not affected. Also, Outlook 2007 is said to work ok in either online or cached mode.
Is anyone able to corroborate this story? This would obviously have a huge impact on planned migrations where Outlook 2003 is still used as a client and cannot be switched to cached mode (e.g. on terminal servers, on older clients with not enough local disk space, on shared or pool laptops etc.).