I’ve left the trail of progress here in case you can’t upgrade, or just don’t want to …
Update 8 Oct 2013: Upgrading to OS X Mavericks GM, ran into some more problems, so I’v been issuing:sudo /Library/StartupItems/VirtualBox/VirtualBox restartto fix it, see also this VirtualBox ticket : https://www.virtualbox.org/ticket/8940.
The symptom is that, upon startup, bridged network guest (in my case, both Ubuntu 12.04 and 13.04) throw an error (which I forgot exactly what now, but something to do with the network adapter). It’ll run just fine in NAT mode.
Any way, thanks thengguy, solution worked perfectly!
I love trying new stuff, so I went ahead and installed OS X Mountain Lion Developer Preview 4. Of note, X11 isn’t included any more. But that’s actually a good thing – you’ll be using the latest and greatest XQuartz by the time you’re finished. If you’re like me, you’re in a rush to fix some program (like Gimp, for example, which is why I wanted to get this fixed right away), here’s the short story.
This is the message you’ll get when you try to run an X11 program, but don’t have X11 because you’re on OS X Mountain Lion. It says you can Continue on to an Apple Knowledge Base Article to get yourself sorted, but all I got was a 404 (and an apology).
The long and short of it is that you now need XQuartz, which was actually the X11 that we in use in OS X to begin with, but now you’ll have to get directly from the XQuartz site. Download and install and *poof* off you go.