I work for a tech consultancy that likes to stay on the bleeding edge, and we recently upgraded to Windows Server 2003 and Exchange 2003, wreaking havoc with my ability to connect to Windows network shares (via Finder or the command line) and to Exchange via Entourage's new quasi-native Exchange implementation.
The fix for file sharing turned out to be that Windows Server 2003 now digitally signs all SMB packets to prevent "man in the middle" attacks that intercept and modify packets. This feature breaks compatibility with all Samba versions prior to 3.0 (still in beta), which includes Apple's Windows file sharing (currently based on Samba 2.2.3a). So to allow Mac, Linux and other clients to connect to shares on a 2003 box, you (or your Windows server admin) will have to disable digital signing of SMB packets in the appropriate local or group permissions area depending on your server setup. After that, the old Finder -> Go -> Connect to Server... works like a charm. Whenever Samba 3.0 becomes stable enough for Apple to use it, you may want to turn packet-signing back on again.)
Getting Entourage syncing with Exchange 2003 (still in pre-release) was trickier because IMAP4 and WebDAV aren't enabled by default in 2003. Again, you or your Windows admin will have to enable these depending on local and group permissions. target=_blank>This document on Microsoft's Mac web site gives some configuration tips. It's still not the missing Outlook X (no task lists, no viewing other people's contacts, only mail folders work as public folders since they have to be shared out by IMAP), but mail folders, calendar, appointments and contacts all sync up automatically with Exchange, which is 98% of what I need.
Also, although I've heard of problems connecting to Samba servers in a Server 2003 environment, I've had no trouble using a Windows machine to connect to either my Mac or our Linux servers via SMB -- although these are all standalone servers which aren't configured to do external LDAP/Active Directory authentication or to act as master browsers or domain controllers.

