The following are the official keyboard shortcuts for Google Plus as of 7/8/11. I will add more as they come in. If you find any please post them in the comments. (Excerpted from the mother of all Google+ Tips Guides)

j: Click ‘j’ to navigate down to the next item in your stream

k: Navigate up to the previous item in your stream

@ or +: mention someone in a post (Guess Twitter didn’t patent the @)

q: While on the Home tab, press qq to move your cursor to the search for people to chat with box.

Space Bar: Scroll down stream

Shift + Space Bar: Scroll up stream.

Enter: Hitting Enter when focused on a Post opens up the comment box.

Tab + Enter: End comment.

Unofficial Shortcuts:
Ctrl – (minus) (a/k/a Zoom Out) on the Circles page makes Circle management much easier. On the Mac it’s pinch to zoom.

Tab: Scrolls through links on a page (this is a Chrome shortcut)