This is new, it was just posted by Google: Doing more with +1 button:
beginning today, we're making it easy for Google+ users to share
webpages with their circles, directly from the +1 button. Just +1 a
page as usual and look for the new "Share on Google+" option.
So how do you do this? Use +Snippets
When you share content from the +1 button... we automatically include a link,
an image and a description in the sharebox. We call these "+snippets"
The real details are here, on the Google Webmaster Central blog: making the most of improvements to Google +.
The +1 button now lets visitors share links to your pages on Google+.
Example code containing each of the +Snippet attributes:
<body itemscope itemtype="http://schema.org/Article">
<h1 itemprop="name">This is the article name</h1>
<img itemprop="image" src="thumbnail.jpg" />
<p itemprop="description">This is the description of the article.</p>
The article suggests using what it calls the configuration tool for +snippets There are several input parameters, but the simplest example was
<!-- Update your html tag to include the itemscope and itemtype attributes -->
<html itemscope itemtype="http://schema.org/Article">
<!-- Add the following three tags inside head -->
<meta itemprop="name" content="Title Of Your +1 Content">
<meta itemprop="description" content="This would be a description of the content your users are +1ing">
<meta itemprop="image" content="images/cool-image.png">