If there isn't a sandbox I'm terrible at SEO. If there is one, I disagree with it wholly.
It's interesting to consider the effect 'sandboxing' might have on small business growth in this country. For example, a startup plummer needs business because he doesn't have a referral base to bring him jobs. He's created a great site and all whitehat SEO. Google may sandbox him for 6 months then actually start putting him in the SERPs. Then he says, great, I'm going to make some design improvements to the site and revamp my link strategies, oops your sandboxed!
I'm just not seeing how any such system would make sense. If you're trying to prevent black-hat from appearing in SERPs, just adjust your algorithms to ignore black-hat tactics?