I got kernel panics after upgrading to 10.8.3 latest betas on my Mac Pro. I know it's beta, but it was so systematic that decided to dig a bit.
The panic log says "Where's my encryption context?\n" and I haven't found anything from Google with that.
Panic can be reproduced in 2 situations: 1) After an update to .3 beta, booting from the non-Fusion main drive (early in the boot); 2) or booting from the Fusion drive in 10.8.3 or 10.8.2 (late in the boot, after login).
After trying things out, it looks like my CoreStorage encrypted Fusion drive Data partition plays some role in it. It's not the primary drive. I use it to run VMWare (Data partition, encrypted) and to make a nightly bootable SuperDuper copy of my main drive (not encrypted). That way I have a 2nd bootable drive if things go bad with the main boot drive, and I can also try dubious hacks on that 2nd boot drive before taking them into actual use.
Panics were annoying, because I wanted to try 10.8.3 in advance, and I wanted to boot from my nightly mirror without a worry. Fusion is nice, because it makes booting from the mirror faster.
Since the panic log mentioned a problem with encryption, I removed CoreStorage encryption from the Fusion drive Data partition, and haven't had kernel panics after that for a few days days. I was able to update the main SSD pair to 10.8.3 beta and the system can now be booted from the non-encrypted Fusion drive boot partition too. All OK with that.
This might be a peculiar bug or feature due to the rest of my setup - the system has lots of hardware, drives and drivers running - but in case you get kernel panics with home-made Fusion drive, try disabling CoreStorage encryption from the Fusion partitions. Decrypt can be started from from Finder or via command line.
The peculiar thing is that I was not able to get 10.8.3 booting from my MAIN drive until I removed CS encryption from the SECONDARY drive. The main drive is neither Fusion nor encrypted. It's a pair of SSDs running in Apple SW RAID1 for capacity and speed.
Not blaming Apple, this isn't a supported setup. Just sharing the observation in case others have trouble with this.