I look into all suspicious posts that I come across, as well as ones that are reported to me.
As I mentioned in another post, you might be surprised how many commercial posts I delete in a given month -- I know other moderators do too.
There are three active "cases" of possible shill profiles I'm suspicious of now. One I have gathered information against that is pretty much irrefutable -- the manufacturer appeared to be posting under the guise of a satisfied consumer of their own products (a much more common practice than you might think, and one that we will be very vigilant about here). It appeared they were doing this here at Head-Fi, and also at Audio Asylum, Audiogon, and the Usenet. Apparently, they didn't understand the concept of IP addresses, because they were leaving footprints everywhere. Because the information I gathered does not technically prove the case 100% yet, I won't mention who the company is (so don't bother asking). But it was enough for me to contact this apparent shill profile via PM, and (gee, what a surprise) they haven't posted here, at Audio Asylum, or at Audiogon again since.
So, long story short, we're watching very carefully. We will do everything reasonable to make sure Head-Fi never resembles the Usenet. And always look at posts that read more like ad copy with great caution. The more I find out, the more I think I'll stick with trusted, reputable companies to buy hi-fi products from.