The Sandbox Effect is the theory that websites with newly-registered domains (or with pages yet to receive a PageRank calculation) are placed in a sandbox (holding area) in the indexes of Google until actual ranking is counted by google.
There are many different opinions about it, including the view that the Sandbox Effect doesn't actually exist, Sandbox Effect are disputed by webmasters, there is general agreement that the Sandbox only applies to search results from Google, and that results from other search engines such as Yahoo or MSN are not affected
You can take a quick look to check of your site is in Sandbox or Not
http://www.seomoz.org/blogdetail.php?ID=701