In tests run on a production environment we received file locking notifications on both Windows and Macintosh systems.
We tested opening a file (a Word document) on a Macintosh first and then opening the same document in Windows while it was still open on the Mac and received a notification that the document was already being used by another user. The notification prompted to open in Read-only mode and then when clicking Save from this document we were forced to do a Save As.
We experienced the same behavior going vice-versa opening the data file on a Windows system first and then opening the same file on a Mac.
The Macintosh in the test environment was Mac OS X 10.5.6 and MS Office 2008 (updated as of 4/3/09) the Windows environment was Windows XP SP3 and MS Office 2007 (updated as of 4/3/09.) The server was Mac OS X 10.5.6 client version using built in AFP and SMB sharing. We authenticated to the server with the same user from both Macintosh and Windows.