Host: Virtual box 4.2.18 on Windows 7 64 bit (quad processor, 16GB RAM) (sorry, this is my workstation at work, and IT doesn't let us have Linux on the desktop hardware, hence my using Virtualbox)
Guest: Ubuntu 13.10 64 bit (x86_64), 2 cpus, 4GB RAM, Guest Additions 4.2.18_88780
I've got a virtual machine with Ubuntu 13.04 x86_64 running nicely. When the software updater in the virtual machine offered to upgrade it to 13.10, I exited the VM, made a clone, tested the clone (all networking, Guest Additions, etc., working just as the original had), then let the updater upgrade the "system" to 13.10. On reboot, the GAs didn't work, but I'm used to that; I removed and re-installed the guest additions using the ISO that comes with Virtualbox, and rebooted again. The video was not working properly and any attempt to access "/media/sf_share" would hang. I did some looking around on line and found the advice to install this from the repository (bad advice, BTW, since it installed 4.2.16, which isn't compatible with 4.2.18, but I digress). This didn't improve things. I re-installed with the 4.2.18 ISO from VirtualBox, and this time the video seems to work, but the hang still occurs when trying to access the share folder.
When I run the original VM that still has 13.04, everything is fine.
I found some forum threads from the Beta period on 13.10, all of which say "of course it doesn't work, it's a beta"; well, this is not the beta, it's the release.
While writing this, I discovered that Oracle has released 4.3 of VirtualBox -- The app's "check for updates" claims no update is available; I may try updating to that and see what happens, but there is definitely an incompatibility between the VirtualBox Guest Additions version 4.2.18 and Ubuntu 13.10.
I'd appreciate any advice for a fix for this without having to update VirtualBox on the lab machines that are not on the network (and I can't change anything that's in the load-set while we're doing verification -- the VM is not part of the loadset, but Virtualbox is.)