From my experience I think that the google Sandbox effect exists. --- Sites with popular keyword terms are dumped from googles index for a period of time. I have built up quality sites.. with some ok back links... and lots of content.... but... still dont list for my main keyword. So do you think it exists? And how long for do you think? As far as I know google hasnt said anything on this topic?
Sure it dose exists, Google Sandbox is a delaying filter to avoid newly created websites from dominating the SERP's however fresh content pages/websites get a push at the very beginning and then suddenly fail to maintain it's position. A lot of webmasters were discussing the same issue since 2004, refer to SERoundTable and WMW for more info about it.
The sandbox is a "thing" of the past. I am launching new client's sites every month and have not observed such since years. Doesn't matter if the market/search term is competetive or not. What looks like sandbox today is just poor SEO knowledge and mistakes on new sites that do not have a lot of inbound links so the mistakes have severe consequences on rankings.
I do think the google sandbox theory exists just to filter newly started high paying niche websites...