In that case, you need to learn more about the Google Sandbox. This is a phenomenon that started in March of 2004. Google refuses to let most new sites rank well for anywhere from 3 to 8 months. The more competitive the keyword phrases you are focused on, the longer the site tends to stay in the Sandbox.
You can read a collection of theories here: Google Sandbox Theories
You probably have not done anything wrong. The initial high ranking is part of this effect. No one seems to know why G teases new site owners with this. It does create an initial euphoria and then a lot of anxiety.
I would not be surprised if your site is doing very well in MSN and Yahoo.