Lets face it, the chances of Overstock choosing a platform that wasn't based on BTC were always pretty slim.
No matter how enthusiastic PB may be about 2nd gen crypto, there is still the little matter of convincing the rest of Overstocks board and shareholders to take a leap into the unknown. CounterParty would be the easiest system to sell to board+shareholders, it can be pitched as a simple step up from BTC.
And XCP is not a bad system, AFAIK, even though I will still maintain that NXT is better
One other useful aspect of this news: one of my concerns about XCP was that their reliance on an external blockchain (ie BTC) could lead to serious issues if/when Bitcoin makes changes to their code. Having Overstock utilising XCP (or an XCP-based system) will give CounterParty a lot more weight when it comes to code changes and maintaining compatibilty of the BTC blockchain with XCP.
Still good news for all crypto, though, can't see a down side yet