Mr. Cutts recently wrote a story about so-called "undetectable spam," in which he countered any paid link building service's claim that it could be truly undetectable to search engine spam sniffers like himself. I'd agree with most of what Matt says - almost any large-scale link development operation that's designed to boost your rankings will eventually stop passing value. This cat-and-mouse game has gone on for years, and even the black/gray-hat folks I know who are the most cautious have reported that Google eventually finds and eliminates the value of what they build.

However, I disagree that all paid links are detectable - there are some that fly under the radar so deeply that even mighty Matt has never found them. Let me give a few examples.

1. Links you buy yourself from the website's owner
   Oftentimes when making a link request, you offer something in return. On many occasions, that something can be a direct monetary exchange. Trust me when I say that even Google's spam hunters don't have time to call up every site owner and grill them CIA-style until they reveal what links people have paid for.
2. One-to-one link building services
   Jim Boykin's crew may be most famous for this service, wherein the SEO will contact relevant sites and offer to buy links or "rent" pages on the domain.
3. Smart link brokers
   I think that every link broker around has two kinds of inventory - the ones they sell to their general clients and the ones they keep for their own sites or sell at a very high rate to their best clientele. This latter type is usually 100% under the radar. It's completely customized on every site, fits in with the content exceptionally well, and is designed so it couldn't be detected even with a very savvy look from an experienced SEO or spam hunter.

The idea behind truly "undetectable" spam is that a reverse link command will appear to be 100% natural. You might look at the links and say, "Man, that guy must have really liked his content to link over there." At that point, the link buyer/seller has won the battle - those paid links really are "undetectable."

p.s. Also see Loren Baker's original post about the new V7N link network for some background on Matt's story.