One practice used by search engine optimization companies that has remained relatively below the radar is the purchase of expired domains with good inbound links that are still active. The general theory is that the Google PageRank is still "alive," meaning that it is not only visible through the toolbar representation but actually still counting in the Google ranking algorithm results of that particular URL. Of course, with Google becoming an approved Domain Registrar (see recent discussion here), who's to say they can't set up a system where domains lose PageRank upon expiration?
The topic comes up occasionally in forums, and yesterday at Cre8asite, someone asked if buying domains can help for link building. Some good discussion so far, including Administrator Bill Slawski (bragadocchio) who points to a recent patent which actually supports the Registrar theory above and says:
I'm not sure how much value there will be to doing something like this in the future, but I wouldn't rely upon it for much for too long.
It would seem that this method may work for now, but all signs are pointing towards the SE's tackling this "loophole" head-on. Please ad your thoughts to the discussion at Cre8asite Forums.