DerivedData is the "build/" Folder that *might* be in your Project's folder if you use the default Xcode 4 settings.

StuFF mc · StuFF mc · commit f06364113fdc · 2012-01-28T12:42:12.000+01:00
diff --git a/Objective-C.gitignore b/Objective-C.gitignore
@@ -13,3 +13,4 @@ build/*
 xcuserdata
 profile
 *.moved-aside
+DerivedData
