CType does more then a static cast; you''d want something more like:

public string CN
{
get
{
return _adUser.Item["cn"].Value.ToString();
}
}

What is _adUser ? Is "cn" a valid index? If so, try changing the line:

get {return (string)_adUser.Item["cn"];}

to

get {return _adUser.Item["cn"].ToString();}

Late post sorry. turns out it was something in my webconfig. DOH!