It has to do with whether or not you've currently got a good connection to Google's servers for sync services and the like. From page 27 of their Android 2.3 Users Guide:

Network status icons turn green if you have a Google Account added to your phone and the phone is connected to Google services, for syncing your Gmail, Calendar events, contacts, for backing up your settings, and so on. If you don’t have a Google Account or if, for example, you’re connected to a Wi-Fi network that is not connected to the Internet, the network icons are white.

I'm not really sure why it bounces back and forth between green and white sometimes. I've noticed it on my phone but everything works fine (I run CyanogenMod, though, so perhaps it has to do with that). It could also be that green means it's currently in the act of syncing, but the wording in the document is a little vague, in my opinion.

In the case of Honeycomb or Ice Cream Sandwich, the colors are blue/grey instead of green/white, but they still have the same meaning.

Edit: For the interested, here are links to the user guide in other languages. The above is the English version.