Obviously you can log off from a PC/Mac but aside from going into your setting and clearing the cache and all data for the app(on the phone) is there anyway to log out of it? Your probably asking why do you want to log out. Well I love the aspect of "circles" you can make and at times I might not have my phone on me when with others. It would nice to have the option to log out of Google+ from your phone so you really know your info you send to certain parties is secure.
In the Google+ app, go to "Settings". In the "Account Settings" section, press the account that you would like to remove. Then on the bottom of the list, there is a "Sign Out" option. Press that to sign out.