If you dig in a bit further, the docs say to use "three, non-negative, period-separated integers" for CFBundleVersion:
http://developer.apple.com/library/ios/#documentation/general/Reference/InfoPlistKeyReference/Articles/CoreFoundationKeys.html#//apple_ref/doc/uid/TP40009249-SW1

However, a quick (and entirely non-scientific) survey of two apps look quite different:

* Photoshop uses "13.0.1.519" for CFBundleVersion and "13.0.1" for CFBundleShortVersionString
* Sublime Text uses "2217" for CFBundleVersion and "Version 2.0.1, Build 2217" for CFBundleShortVersionString

I don't know where CFBundleVersion is used. I *do* know that CFBundleShortVersionString is used in the Finder "Get Info" dialog and when right-clicking and selecting "Open With >" (the number is in parens after the app name)