|Photo by Matthew W. Jackson (copyright)|
It looks like some sneaky webmasters (or specialised companies) found a way to do it... by spamming other blogs with messages that are unlikely to be removed: they post messages such as the ones below full of compliments with a link to their commercial website. These messages increase their rank in search engines.
This techniques even has a name: Spamdexing (see Wikipedia).
If you own a blog and receive comments containing text with a link, I recommend checking if it adds something to the discussion and otherwise delete it.