Mine has always been doing that ever since I started using Leopard (different builds).

diskeeper, I replaced the file but still no luck with the icon showing up.

One thing I noticed is that on the toolbar where the volume icon is suppose to be, there is just an empty space. The icon itself is not visible. As soon as I uncheck the "show volume in menu bar" option in Sound, that space disappears.

diskeeper, I got it work by replacing that file. I had to replace it through safe mode though.