I have not been able to fully test Snow Leopard’s built in Exchange support since we are not running a full 2007 environment. But as Tim Anderson points out, Snow Leopard now uses Exchange Web Services to provide a more seamless presentation. Although not everything is perfect, as Tim points out.

You will be able to connect on an internal network or VPN, provided that EWS is enabled, which it usually is. You may need to install a digital certificate to avoid warning messages.
Mail, Calendar (iCal), tasks and notes in your Exchange mailbox all appear nicely.
When outside the network, you will only be able to connect over the Internet if EWS is published externally, which it often is not. You cannot use RPC over HTTPS.
There is no access to public folders (note that these are deprecated, but still widely used).
It is not possible to send from an email address other than the default.
You cannot use Exchange delegation features, such as accessing other mailboxes.
Mail will download the entire mailbox; you cannot set it only to download recent items. There is no “online mode” as there is with Outlook.
When offline, you can access existing items, but new messages have to be saved as drafts. This is unlike Outlook, which gives you full access to send mail, delete etc, and synchronises on re-connect.

It will be interesting to see how things shape up when Outlook for Mac is released. Will it use Microsoft’s proprietary MAPI protocol or EWS like Snow Leopard does?