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Typically they'll submit links in the "Link" or "Name" field.

But as you say the majority of spam submissions will actually include a link in the body of the comment too.

(Actually many comments include the link with numerous variations: BBCode, HTML, and even Markdown formatted. As a heuristic it is easy to write off spam that uses three or more different linking stratagies - I do that in my blog-spam detection service @ http://blogspam.net/ )

I see what you did there ;-) But thanks for the explanation

spammed this post about spam with his anti-spam service?

edit: wordpress plugin looks perfect for a few of my blogs

Many times they post just to see if the blog has auto approval or not. Or if the blog has auto approval after 1 approved post they'll post an innocuous one then later come back with the links once they'll get auto approved.

Yes, the comment author links the page in their profile or sig (depending on the platform), posting generic praise comments is a huge chance they won't be deleted.