Apple’s developer website only links to the latest version of Xcode, which requires Snow Leopard (Mac OS X 10.6).
If you’re still running Leopard (10.5), go to this link after logging in with your ADC credentials:
http://connect.apple.com/cgi-bin/WebObjects/MemberSite.woa/wo/9.1.17.2.1.3.3.1.0.1.1.0.3.3.3.3.1
Search for “3.1.4″ — it’s 1/3 down the page. You might also want to check the wikipedia list of xcode versions to see if 3.1.4 is still the latest version.
(November 2010 update: Apple’s URL changed)