I am an enterprise customer and can log into the production environment through the API.  Today I created a sandbox (I guess developer version) of my production environment.  I am unable to log into the sandbox version.  I logged into the sandbox version through a web portal and generated an sercurity token.  I am using username.sandboxname and passwordtoken.  Here is the code:

        private bool Login(string strUsername, string strPassword, string strToken)
        {
            // Timeout after a minute
            _sforceService.Timeout = 60000;

            // Try loggin in
            LoginResult loginResult;
            try
            {
                loginResult = _sforceService.login(strUsername, strPassword + strToken);
            }
            catch (SoapException e)
            {
                return false;
            }

            if (loginResult.passwordExpired)
                return false;

            // Reset the endpoint of the service to the virtual server instance that is servicing our organization
            _sforceService.Url = loginResult.serverUrl;

            // Set the persistent SOAP header.
            _sforceService.SessionHeaderValue = new SessionHeader();
            _sforceService.SessionHeaderValue.sessionId = loginResult.sessionId;
            return true;
        }