That is what Jason Cohen (CTO of WP engine) is saying at blogs.csoonline.com
He also said: (what the pingback function does, calling it “an altruistic, friendly, social system.”)
What can happen is a lot of pingbacks with the links in the comments. If done on many pages with a lot of effort it can happen. I recommend everyone use moderated comments to prevent automated bots if you must turn on pingbacks.
And remember – you can always turn off pingbacks. In Settings–> Discussion Settings
we have no pingbacks in our WordPress sites.