There are [ODBC and JDBC pieces for Mac OS X and Mac OS X Server|http://stackoverflow.com/questions/3452/sql-client-for-mac-os-x-that-wor ks-with-ms-sql-server], so yes, Mac clients and servers can connect to Windows SQL Server databases. [iODBC|http://www.iodbc.org/dataspace/iodbc/wiki/iODBC/ODBCMacOSX] and [OpenLink|http://www.openlinksw.com> might be worth a look; there are open-source and commercial products, and [Apple has some older tools|http://support.apple.com/downloads/ODBC
Administrator_Tool_for_Mac_OSX], or launch Terminal.app and enter +man iodbctest+ and have a look around.
If you're connecting the other way (from Windows clients into Mac) you'll have to look at the details of the particular database you plan to connect to. MySQL offers remote connections, for instance.
If you're planning on database replication or a single database that is spanning platforms, that gets rather more hairy. You'll probably want to pick one host platform and one database package and use whatever it offers for replication. A mix of platforms and databases isn't likely to work.
And FWIW, there's not really a concept of a "Mac network" or a "Windows network". And you can freely mix Windows and Mac OS X and OpenVMS and Linux and BSD and... together on the same IP network.
There's an "IP network" (as well as a few "SNA network", "DECnet network" and various other network protocols that are rarely observed in the wild), and there are "Mac services" (example: Podcast Producer) and "Windows services" (SharePoint) and "generic services" (DNS) available.
