They do not write anything about licenses at http://developer.android.com/guide/practices/ui_guidelines/icon_design.html#menuapx (Menu Icons)
I guess they want to have standard icons look the same in all apps, so users dont get confused. Right?
Now I would love to use the Menu-Icon "see map" (a folded map) from the Google Places App. Where can I get it for a commercial project? (Its not in here)
Places is a proprietary app by Google, just like Gmail, Maps, or Voice. These apps are not open-source, so you won't be able to go and find their icons in the git hub.
BAD IDEA: You could always take a screen shot of the app using DDMS and create your own icon from that, but you might be running some legal risks (since the app isn't open-source). It'd be like using the Photoshop icon for your app.