Oh sorry, I must’ve missed all those ads for a WS2003 update. No, I’m pretty sure this was advertised as a new OS.
Whatever the marketing is, the truth is that Vista is based on the Windows Server 2003 code, which was based on the Windows 2000 Server code, which was based on Windows NT 4 code, which was based on Windows NT 3.51 code, etc.
It's just another update to the NT line, like OS X Leopard is just another update to the OS X line.