Just been advised that article 905803 which discusses the impact of high item counts on Outlook performance has been updated and re-released for Exchange Server 2007 and Outlook 2007;
'Outlook users experience poor performance when they work with a folder that contains many items on a server that is running Exchange Server'
Of course we are still using ESE and the story concerning item counts is basically the same as with Exchange 2003. i.e. keep item counts down in your critical Outlook folders. The difference is that with E2K7 we should be able to deliver far bigger mailboxes to our user community and therefore the need to understand the impact of item counts on Outlook performance before we deploy, so that we can plan accordingly, is even more critical. Good planning will ensure that Outlook performance does not become prohibitive to a good user experience.
The basic premise of the solution to this issue is to ensure you plan your E2K7 server infrastructure appropriately for performance and capacity but also that you manage your mailbox. As a rule of thumb it is recommended that..
"..you maintain a range of 3,500 to 5,000 items in a folder depending on the capacity of the Exchange Server environment. Additionally, you can create more top-level folders or create sub-folders underneath the Inbox and Sent Items folders. When you do this, the costs that are associated with index creation will be greatly reduced if the number of items in any one folder does not exceed 5,000."
The article is well worth a quick read - especially the 'more information' section which has some good technical detail.