bug in Software Update?
Running Mountain Lion OS X 10.8.5 (12F45), and App Store Version 1.2.2 (129.16). The "App Store" app shows an available update in the Dock Icon (i.e. a 1 inside a red circle), but there is actually no applicable update to be downloaded. Running "software update" does not clear this indicator.
While this is hardly the most earth-shattering bug in the universe, perhaps the Apple folks will read this report and fix the problem.