[BlackTiger]

YES! It's working again in 2.3.0 final. But maybe some feature can be added. I will state it below
I did it like this:

Under this line:
if (md5($loginpassword)!=$bbuserinfo[password]) {

add:
$ipaddress=iif(getenv("REMOTE_ADDR")!="",getenv("R EMOTE_ADDR"),$HTTP_HOST);
$iphostname = @gethostbyaddr($ipaddress);
$message="Someone is trying to login using your admin account!\n\nUsername he tried to use: $loginusername\nPassword he tried to use: $loginpassword (".md5($loginpassword)." in encryption)\n\nThe IP address is: $ipaddress\nThe host is: $iphostname";
mail($webmasteremail,"Warning: vBulletin Admin Login Tried",$message,"From: \"$bbtitle Admin CP\" <$webmasteremail>");

It works in 2.3.0 final, but only when the username of the admin is used.
So if you don't have the username Administrator as admin, you won't get any mail if somebody is trying to login as Administrator.
If you username is Joe and you are admin, and somebody is trying to login as Joe, you -will- get an email.

Feature:
A nice feature would be if could be detected if *anybody* is trying to login to the Admin cp no matter which username he is trying, so you will also get an email if somebody is trying to login as Administrator in your admin cp, even if the user "Administrator" does not exist.