The original post (above) was posted in July 2011. It's now May 2015.
Things sure have changed for businesses and Google+ with the launch of Google My Business
in late 2014.
Google My Business
finally integrates everything Google+
with Business listings (citations), reviews and much more.
The crucial thing to understand is that your directory listing in Google My Business
becomes your "Gold Standard" for ALL online directory listings ó and Google will factor them all into calculation of your SERP position.
Learn more ó and download my Fast Start Guide
(free, no signup) from this post: