Many people get this wrong.
Definition of the sandbox effect: You can consider your site sandboxed by google if it ranks well in Yahoo and MSN but is nowhere to be found in the Google SERPs.
New urls tend to get a fresh boost. Then, google's algo will decide your real position. You need to get the right amount of backlinks. Too few and you wont beat the competition. Too many, and you'll raise a red flag and google wll throw you in the sandbox, never to return.
Either way, you won't know where you stand with G for at least a few months.