I'm not sure how Sony is marketing PS Plus in the US, specifically the instant game collection. But if it is pushing it well and making people aware of the value then i think its quite a big factor.It's a particularly good deal for new PS3 owners.
Not to mention that this year's 199USD bundle is much better value for money considering the 4 free games that comes with it.
Bottom line: There's never been a better time to buy a PS3.