I have been having a problem with Ubuntu 11.04 and Unity when run as a client OS within VirtualBox. The problem is that I could not get the Client Guest Additions ISO to mount. The thing is, though, that they mounted at one point because you need those extension to run Unity under VirtualBox.
So today when I upgraded to VirtualBox 4.0.10, I decided I would try to figure out what was going wrong. I eventually logged out of Unity and went back into Gnome and sure enough, there was the ISO listed in the Locations menu. But back in Unity, the ISO was not anywhere. I finally opened Nautilus, went to Edit | Customize Toolbar and added Computer to the toolbar. Then I clicked the computer icon in the toolbar and there is the ISO and the hard drive. I double-clicked the ISO and it them showed up both on my desktop and in the Unity menu.
I hope this helps other souls out there that couldn't figure out where their mounted ISO was...
Normally CDs, USB sticks etc. when mounted on Ubuntu, show up in the /media directory. You can go there with Nautilus (press Ctrl-L and enter /media in the location bar) or in a terminal window with cd /media.
Sorry, but I looked in /media and the ISO/disc was not there. I also looked under /mnt and /cdrom. Not there either. However, after I went through the steps outlined in my post, then the ISO/disc showed up under /media. I don't know if this is an issue with the VirtualBox Guest Additions or not. The "shared folders" that I set up in VirtualBox show up under /media and are available immediately. And under Gnome accessing and ISO/disc wasn't an issue because I could see it in the Places menu. It was only in Unity that I had a hard time locating it; maybe it is somewhere but I sure couldn't find it.
Note that this is what I experienced under VirtualBox. A physical machine might behave differently.