It was on by default in Windows Server 2003 SP2, so if your NIC supported Offload, or RSS, or that other thing I can never remember, it was enabled.
But: we (PSS we) typically turn it off as a first troubleshooting step for any network-related issue –
a) because we know from experience that several drivers seem to do interesting things with it installed (that’s a nice way of saying update your drivers),
b) because several of our drivers do interesting things with it (if you’re going to choose to use it, check for recent-model tcpip.sys hotfixes), and
c) because we want to be able to see TCP traffic in a network capture for troubleshooting purposes.
Off-unless-opted-in brings parity with Windows Server 2008.