It comes down to the definition of "sandbox". If a new website meets a certain set of criteria it ranks poorly for a period of time. The Google algo does this and so does Yahoo, only using different criteria. It does not happen all the time on either.
Here's a case study. Last October we launched a new site on a new domain. It had about 200 unique content-rich pages and filled a niche not yet filled by other sites. The main keyword was in the site name and moderately competitive. Within a few weeks we had links from over 100 websites, including many of the important sites in its field.
In less than 2 weeks the site ranked #5 on Google and has stayed in the top five since. Somehow we avoided the "Google sandbox". However the site currently ranks #73 on Yahoo. I've tweaked on page factors and it doesn't seem to effect the rank on Yahoo by more than a few slots.
Maybe our highly successful link campaign triggered the "sandbox" effect. We didn't break any seo rules that I know of. It does not seem to be a penalty as it ranks #1 for the site name which is "Keyword Word Word". Yahoo has found most of the backlinks and indexed most of the pages on the site, so it's not slow indexing.
I'd be interested if anyone has had similar experiences. And even more interested if anyone knows how to stop this happening. :-)