I'm using Git for Xcode 4 project version control. I've explicitly added ProjectFolder.xcodeproj/project.xcworkspace/xcuserdata/myUserName.xcuserdatad/UserInterfaceState.xcuserstate to .gitignore, but Git it won't ignore it. Any ideas why this is so?

Git is probably already tracking the file.

From the gitignore docs:

To stop tracking a file that is currently tracked, use git rm --cached.

Use this, replacing [project] and [username] with your info:

git rm --cached [project].xcodeproj/project.xcworkspace/xcuserdata/[username].xcuserdatad/UserInterfaceState.xcuserstate
git commit -m "Removed file that shouldn't be tracked"

Alternatively you can use the -a option to git commit that will add all files that have been modified or deleted.

Once you've removed the file from git, it will respect your .gitignore.