abates - 10:01 pm on Aug 24, 2006 (gmt 0)

Net::Trackback is a library of functions used to make programs which send trackback pings to blogs. What you're probably seeing is an application searching your web page for URLs to ping. Since you've got hits coming from different IPs, it's most likely a spammer looking to send spam pings.

If your site does not support trackbacks, you can just block this user agent. If it does, you may want to block it anyway to avoid the spam. :)