Apple's X11.app was never part of Xcode, though it may have been distributed on the Dev Tools discs (maybe? I can't recall off the top of my head).
The X11.app package for 10.5 is on the 10.5 discs, somewhere, though not installed by default. I can't tell you exactly where to find it, but if you put the disc(s) in a running system, it's not difficult to find and install.
Xnest is still included, though it has a bug causing you to have to type space twice to register one on the X client. It's pretty annoying, and Apple's stance on fixing it (not fixing it) is even more so: "Maybe in 10.6"