Seoroundtable recently disabled their trackback facility due to trackback spam and comment spam. Some black hat SEO has been using this technique to get links and increase PageRank.

Trackbacks has been a very good tool for linking threads and referencing other people's post in your blog. These days, however, it too became target for spam.
Fortunately for wordpress users, there is a free tool (free for personal use) that can assist in handling comment and trackback spam. It is the akismet wordpress plugin.

If you're making money out of your blog (more than$500/mo.), then you need the problogger license which costs only $5/mo. I think that's fair enough.