I've written extensively on this subject already. Probably my best statement on this question is at GoCertify.com, where I wrote an article entitled "Microsoft Certification: Windows 2000, Upgrade, or Windows Server 2003?" for the mid-October "Certification Watch" newsletter.
The quick answer to your questions is, if your current or prospective employer(s) don't plan to upgrade to Windows Server 2003 for more than a year, stick with Windows 2000. Otherwise, go for Windows Server 2003. And, as for when they might retire Win 2000 MCSE, the smart money says that won't happen until the server platform that ships after the next desktop--code-named "Longhorn"--ships. This isn't supposed to happen until 2006, and the next server probably won't hit until 2007 as a consequence. That means 3-4 more years of life for the 2000 exams, since MS has promised not to retire them until those releases occur.
This was first published in December 2003