Interestingly junk like this gets indexed and ranked pretty highly like google.
I've come here a long while after the BS ensued, only to find a bunch of schmos claiming that there's no similarity at all between WordPress MU and WordPress, which frankly is a load of crap.
This is yet another case of the clueless masses picking on someone in need of help, by focusing on a single tiny reason not to help them.
If any of you had known the answer, any one of your posts may have taken fewer keystrokes and far less aggravation to actually resolve the problem, but instead you take solace in being technically right, instead of being socially forgiving.
Nice one. You should be proud of yourselves.
Now, to answer the question in plenty of detail, for anyone else tripping over this in their Google results:
What the OP has seen, and is accustomed to, are called "Pingbacks" which are a WordPress-specific feature that is similar to Trackbacks, but does not require any effort beyond linking someone else's post in your own.
Trackbacks are compatible with other blogging tools, and the URL they need in order to send you one is http://yoursite.com/d/a/t/e/your-post-title/trackback
If you try hitting this link manually, it will direct you to your post and remove the /trackback from the end. That doesn't mean that the permalink and the trackback link are the same. The latter with 'trackback' included, is the link users of other blog software will require in order to send you a trackback.
Now, the reason the OP is seeing lots of trackback hits on his older blog posts is trackback spam. Spammers crawl the web looking for posts they can spam, and the older they are the more likely they are to be indexed extensively.
Hope that helps the next person along.