You might want to post the actual error. This tells me nothing as I know the UI2DSprite and NGUISettings both have the 'sprite2D' properties.
Sorry about that. Here's the error code.
Assets/NGUI/Scripts/Editor/NGUISettings.cs(385,19): error CS0029: Cannot implicitly convert type `Sprite' to `UnityEngine.Sprite'
Edit:Didn't want to bump the thread.
Thanks, I found the problem thanks to your explanation.