Afternoon, just thought I'd mention there's what looks like a small copy & paste bug in the latest version of UIScrollBar.cs.
Line 182 of UIScrollBar.cs in the alpha property to be exact:
NGUITools.SetActiveSelf(mFG.gameObject, (mFG.alpha > 0.001f));
should be
NGUITools.SetActiveSelf(mBG.gameObject, (mBG.alpha > 0.001f));
The current version is never setting the scroll bar background as active so the it never displays in-game.