The Google Sandbox was a term coined back in 2004 when SEOs and webmaster began noticing their new sites were not ranking as quickly as they use to. In short, your new site got put into a sandbox, where you waited, until you would rank.
That has not been talked about much in years but there are always some webmasters and SEOs that bring it up. Truth is, new sites can rank fast in Google these days, if they meet Google's algorithms requirements.
That being said, a Black Hat World thread has some SEOs and webmasters complaining it is taking longer than usual to rank new sites in Google.
If you look past all the bickering and yelling in the thread, there are many SEOs and Webmasters, likely on the black hat/churn and burn side, agreeing that it is taking longer.
Can this be a new Google Sandbox or just a specific case of some webmasters not building sites right and expecting them to rank well?
I have not seen many complaints elsewhere but I wouldn't expect to see that in normal forums.
Forum discussion at Black Hat World.