>snow leopard doesn't have a MAPI connector
Macs have never natively had MAPI since MAPI rather relies on Windows APIs that the Mac obviously does not have... and the only MAPI client I am aware of was Outlook 2001 (which predated OS X). Later versions, and Entourage, use webDAV. Oh, and Macs came with an IMAP connector for their Mail.app software
Snow Leopard comes with an EWS connector instead of IMAP (although I assume iot still includes an IMAP connector). And the latest version of Entourage use EWS instead of webDAV, rather than instead of MAPI (although I'd assume that it still includes webDAV).
One problem you would have in blocking Macs connecting via EWS is that I'm note sure that there is a way of identifying the application or platform making use of EWS. About the only thing you are guaranteed to know are the user credentials.
>microsoft is eventually going to give mapi the axe
Not any day soon. And certainly not during the lifetime of Snow Leopard ...
>Snow Leopard comes with a MAPI connector out of the box according to the blurb
Sadly not, Zel, The blurb I've seen simply says it has out the box support for Exchange Server - which isn't quite the same thing; a more close read shows that that is support for Exchange Server 2007, and that that support is provided through EWS ...
>There also historically been some pretty big issues around mac's causing high cpu usage on exchange
Have to say I have not been aware of Macs doing this. There was an issue with IMAP clients (of which Macs would be an example) causing high CPU usage, but this was actually a problem at the Exchange end. But we don't use a lot of Macs, so I may have missed this issue.