I have been successfull at impersonating users in another domain, but only with a trust set up between the 2 domains.
var token = IntPtr.Zero;
var result = LogonUser(userID, domain, password, LOGON32_LOGON_INTERACTIVE, LOGON32_PROVIDER_DEFAULT, ref token);
if (result)
{
return WindowsIdentity.Impersonate(token);
}