Virtualbox - Install Guest Additions over again every time Linux kernel updates

You can recompile the kernel modules with

/etc/init.d/vboxadd setup

That way, you don't need to reinstall them - just rebuild the modules after a kernel update.

Ubuntu provides the guest utilities from the open source edition of Virtual Box. As far as I know, there is no difference between the two editions as far as the Linux guest utilities are concerned (for example, with a closed source edition of VirtualBox, you can access USB devices from a Linux guest even if the VM has the open source guest utilities).

The packages are virtualbox-ose-guest-utils, virtualbox-ose-guest-x11 and virtualbox-ose-guest-dkms. With that last package installed, the necessary kernel modules are compiled automatically when you upgrade the kernel.