Safari 5.1 shipped with WebKit 2. We should update the Debugging WebKit page <http://www.webkit.org/building/debug.html> to describe how to support debugging WebKit 2.
Here is how to do it for people that do not know yet: defaults write com.apple.Safari IncludeInternalDebugMenu 1 And then in Safari 5.1 you'll see a Debug menu. From here you can either do "New Single-process Window" or turn off "Use Multi-process Windows". Once that is done your Xcode breakpoints will be hit.
The better way to do it is here: https://trac.webkit.org/wiki/Debugging%20the%20WebKit2%20Web%20Process
The linked wiki page is empty and the content appears to be on debug.html now. Closing bug.