The A20-kernel-3.4.43.tar.xz was found on cubieforum at this URL and recompressed from gz to xz :,755.0.html
This is a working kernel for A20 I personnaly tested and use sicne 19 sept 2013 (don't know until which day).

I replaced with it the default kernel in lubuntu quantal card version found on here. I had lot of crash with this kernel.

the µSDcard image is on the HTTP, here : cb-a20-lubuntu-desktop-card-v105.img.gz (md5sum file)

To copy it on SDcard, plug your card on your reader, umount it and (choose the good /dev/sdX device file, Warning, will delete everything, don't delete your system or data filesystem !!!!):
zcat cb-a20-lubuntu-desktop-card-v105.img.gz >cb-a20-lubuntu-desktop-card-v105.img
sudo dd bs=2048 if=cb-a20-lubuntu-desktop-card-v105.img of=/dev/sdX
To replace the kernel :
* plug the µsd card (it will be automagically mounted)
* Unarchive the package and copy the content like this (xz needed, on old system try 'xzcat file.tar.xz | tar xf -' instead of 'tar xf file.tar.xz'):
tar xf A20-kernel-3.4.43.tar.xz
cd A20
mv /media/usb0/uImage /media/usb0/uImage.default
mv /media/usb1/lib/firmware /media/usb1/lib/firmware.default
mv /media/usb1/lib/modules /media/usb1/lib/modules.default
cp -a boot/uImage.3.4.43-sun7i+  /media/usb0/uImage
cp -a lib/* /media/usb1/lib/
umount /media/usb0
umount /media/usb1
plug it in your cubieboard, reboot and enjoy !!