Similar to this previous question, but I want to enable the battery full notification sound, not the opposite :)
My Galaxy S used to make a nice beep and switch the screen on briefly when the battery was fully charged, since the Gingerbread update (official Samsung 2.3.3 JV9 release) it still briefly switches the screen on, but no longer makes the noise.
This used to be great because the plug socket on my desk is right at one end, so (if I don't want a long cable getting tangled in everything) the phone's out of my line of sight and I rely on the audio notifications to tell me there's a reason to glance over at it. Now, since Gingerbread, I don't get any audible notification for the battery full, and never know when it's been fully charged.
Unlike a lot of people here I don't plug my phone in to charge overnight (why would I, when I have to pay for the electricity at home :D), so was never bothered by the noise waking me up.
I've hunted through all of the Sound and Notification settings that I can find, and already have System and Notification volumes turned up, so it's not those. I'm slightly worried that they've listened to the moaners and removed a useful feature, but hope that buried somewhere there's a nice setting that can keep everyone happy.