Running 12.04 for the first time today, I still have the old login sound. (Well, technically I don't have any login sound at all, oddly. Nothing in startup apps for it. When I do a file search for Gnome Login, I find a sound file for it but it's the old sound.) I was looking forward to the new sound, are there still plans to implement it?
I managed to change it;
First, record the startup sound you looking for: http://www.youtube.com/watch?v=yJw178Iunk4
You could convert the video to MP3 (youtube-mp3.org), and edit the sound file. Export it to an MP3, an place it in /usr/share/gnome/autostart.
Then follow these steps: http://www.upubuntu.com/2012/03/how-to-play-custom-login-sound-mp3-on.html
This is for 11.10, but on 12.04, that worked for me! This haven't made the boot time slower or anything like that.