Heard a lot of talk about this on the radio today, including an interview with TMZ's Harvey Levin. Say what you like about TMZ, but Levin is a smart guy.
1) It is possible that the police simply missed finding the thing. Yes, they combed the area, but there are still limitations.
2) The ex-cop who held it all these years is an idiot, and almost certainly knew he shouldn't have done it.
3) The LAPD is trying to track down the construction guy who dug it up
4) The ex-cop actually refused to give the knife to the LAPD last month. According to Levin (take this for what it's worth): "he was a real jerk about it." They actually threatened him with a search warrant, so he gave in.
5) After all of the above, a report I heard later on is that initial, unconfirmed word is that this knife is not consistent with what we would expect from the actual murders.
6) If the knife is the murder weapon, which it doesn't look like it is but bear with me, it would sure be interesting if they found DNA/fingerprints of the victim(s) and OJ and someone who (potentially) disposed of the thing. Believe it or not, they technically could go after someone for Accessory to Murder (even though OJ was acquitted), but realistically that will never happen. So no need to worry, Mr. Cowlings.