In version 0.6 of the unity3d plugin there's an issue where Unity compilation errors don't seem to be caught in certain situations. In a recent build we had the following error text but the build was marked as a success:

Compilation failed: 1 error(s), 0 warnings
----CompilerOutput:-stderr---------
...removed...
----EndCompilerOutput--------------

• Finished compile Library/ScriptAssemblies/Assembly-CSharp.dll
  ...removed...

Error building Player because scripts had compiler errors