This question partially addresses my question, but is not comprehensive enough for my XCode 4 needs.
I don't really understand the .xcodeproj structure but it's clear not everything should be in SVN. I previously had only MyProject.xcodeproc/project.pbxproj committed but another developer has committed a whole stack of files - adding xcuserdata and project.xcworkspace subdirs which both contain a lot of files named _xxx.
For xcode4, is there a well-defined list of which files should actually be considered "the project" and be in a multi-developer source control system, and which are user-specific files which should not be?