Steps to reproduce:
1) Set Xcode's DerivedData location to be relative.
2) Build an empty iOS project in Unity.
3) Open the build in Xcode.
4) Build the project in Unity on top of the old one.

Result: the build fails:
IOException: Directory /Users/vaidas/Downloads/eeveve/test/DerivedData/Unity-iPhone-ckfnsrzreliwhndtbdnhsjmtoxbo is not empty
System.IO.Directory.Delete (System.String path) (at <a8ed250850854b439cedc18931a314fe>:0)
System.IO.Directory.RecursiveDelete (System.String path) (at <a8ed250850854b439cedc18931a314fe>:0)
System.IO.Directory.RecursiveDelete (System.String path) (at <a8ed250850854b439cedc18931a314fe>:0)
System.IO.Directory.RecursiveDelete (System.String path) (at <a8ed250850854b439cedc18931a314fe>:0)
System.IO.Directory.Delete (System.String path, System.Boolean recursive) (at <a8ed250850854b439cedc18931a314fe>:0)
UnityEditor.iOS.PostProcessiPhonePlayer.UpdateInstallLocation (UnityEditor.iOS.PostProcessiPhonePlayer+ProjectPaths paths, UnityEditor.iOS.PostProcessiPhonePlayer+BuildSettings bs, UnityEditor.iOS.PostProcessiPhonePlayer+IncludedFileList includedFiles, UnityEditor.Build.Reporting.BuildReport buildReport) (at /Users/vaidas/trunk/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:1354)
UnityEditor.iOS.PostProcessiPhonePlayer.PostProcess (UnityEditor.iOS.PostProcessiPhonePlayer+BuildSettings bs, UnityEditor.iOS.PostProcessiPhonePlayer+ProjectPaths paths, UnityEditor.RuntimeClassRegistry usedClassRegistry, UnityEditor.Build.Reporting.BuildReport buildReport) (at /Users/vaidas/trunk/PlatformDependent/iPhonePlayer/Extensions/Common/BuildPostProcessor.cs:730)