To avoid pingbacks when linking internally to my blog---to an earlier post, for example---I have successfully used an abbreviated URL: /2008/04/12/scott-hamilton/ instead of http://leisureguy.wordpress.com/2008/04/12/scott-hamilton/. This worked fine, but the new interface completes the URL: I enter "/2008/04/12/scott-hamilton/" and when I publish the post, that has been augmented to "http://leisureguy.wordpress.com/2008/04/12/scott-hamilton/", and I get the internal pingback when someone clicks the link.
I contacted support, and the workaround is to go to HTML view before clicking "publish" and delete the "http://leisureguy.wordpress.com" by hand, and then immediately publish.
So far, so good. BUT: when I edited a lengthy post that had lots of internal links, suddenly all those URLs were completed, and it was a tiresome project to go through the entire long post looking and trimming back the URLs by hand.
I suggest that the autocompletion feature be abandoned. It turns out to be much more trouble than it's worth. Probably if the URL is not complete, the writer didn't want it to be complete.