I run a weblog based on pMachine (www.simulation.cc/weblog.php). About ten days ago it started getting hit regularly by a group of sites (poker, bayfront homes,  etc.) I thought it might be comment or trackback spam, but they don’t try - or at least don’t succeed - in leaving either comments or trackbacks.
Now, having banned these domains from using my site (through my ISP), I’m getting a lot of the same from other URLs which seem quite genuine, all of which quote the poker, bayfront homes, etc., sites as the referrer. (But when I look at the supposed referring pages, they don’t have any links to my site.)
Can anybody advise me what’s going on? Nothing is happening apart from a lot of hits on the site:  these are not enough to cause any problems so far - my weblog is good but it’s not yet quite as famous as Lawrence Lessig or Joi Ito. The site isn’t being defaced. As far as I can see nothing has been added or changed on the site files.
Is there something I missed?

It sounds like they’re trying to “referrer spam”.  pM Pro doesn’t have anything that displays referrers, so there’s no benefit to them to do it, but I’m sure it’s all automated anyway.  My suggestion would be to block any incoming referrers from those “bad” sites.  Try a Google search for something like .htaccess block referrer.