As far as I know, this is a common occurrence with certain sites. They may initially rank well for a brief period of time right out of the gate and then be ‘Sandboxed’ so to speak. This happened to several of my sites, but it didnt last long.
The other thing some people overlook is that the ‘Sandbox’ applies to Google only. You may very well have some great results in MSN very quickly. Unfortunately the traffic that results from many top postions in MSN–although better than nothing–is quite on the low side, at least for me, when compared to comparitive SERP results in Google.
Keep us posted and let us know your URL!