I can confirm that Ubuntu 12.10 is NOT WORKING with HP Compaq 6820s anymore.
12.04.1 LTS works fine.
I tried boot from USB and DVD, just cursor blinking after loading initrd.
Something is broken, i suspect that KMS is causing the problem, it already causes some minor problems on previous versions on the Ubuntu ( USB mouse stuttering and freezing ).
EDIT: I just have tried boot Ubuntu 12.10 with "nomodeset" option enabled, and it worked ! You can try "radeon.modeset=0" also, it should do the same as "nomodeset" option. Sad thing is, that HW Accel 3D is completely disabled with that option, just 2D works.
So, if anyone can fix this. Thx !