Xcode 3.2.x is documented as only being for OS X 10.6.x (Snow Leopard). As far as I know, Xcode 3.1.4 is the most recent version of Xcode for OS X 10.5. While it seems to be hidden away, I found it was possible to get to the old links for it, if you are registered at the Apple Developer Connection website, by using this link:
http://connect.apple.com/cgi-bin/WebObjects/MemberSite.woa/wa/downloads
and then clicking through to the "Developer Tools" section and scrolling down to the download link for "Xcode 3.1.4". I'm not sure why Apple has made it that hard to find older stuff. Perhaps they are trying to get people to join the paid Mac Developer program?
UPDATE: I see this is the same link that Evan's suggestion eventually gets you to. They could make the links a little more obvious, especially from the Xcode page itself!