I warned people about this when I first started seeing them using Google+ Comments on WordPress posts (or using the new Google+ Comments system on Blogger blogs).
I set up a test WordPress site and added the Google+ Comments system to it.
After leaving it running for a week I notice it is already attracting spam and after checking out some of the spammers see they are also targeting popular Google Plussers who have installed Google+ Comments on their blogs/sites.
Here is just one example https://plus.google.com/117260596227003552028/posts and here is the spam on louisgray.com – http://blog.louisgray.com/2013/05/mobile-data-goes-up-phone-calls-go-down.html (it may have been removed now as I have already reported it)
A quick tip for those of you who are using Google+ Comments, go to Google+, search for your domain, set results to recent and check that spammers are not targeting you.