Guys I'm having the same problem. After a Kernel Panic, which actually was triggered by Intego's Proteciotion software, The computer would go white after booting, staying so forever.
I tried with a safe boot, and it works. But it sticks to using the Discrete Graphic card and never switches to Integrated. Even trying to force it to do so via gfxcardstatus application.
Moreover using it in safe mode is really weird the way it acts with graphic transitions (as when using gestures for Launchpad or Spaces), showing extremely slow graphic redraws.
My setup is MacBookPro early 2011 15" used in clamshell mode with an Apple Thunderbolt Display.
Now, luckily I kept a bootable disk of my previous Mavericks installation and it's working fine. Switches between gfx cards normally, no graphic glitches, no slowdowns, etc.
The only weird thing that happened is that it wouldn't boot initially. While before I was able to boot with it fine without any problem. At another attempt to boot from it, I got ResetPassword (Yosemite version) loaded without me opening it (!!!) and it asked for decrypting the Mavericks disk (file vault 2 protected).
So I had to insert the file vault pwd to have the disk decrypted and then it started to boot.
I'd originally did a clean install of Yosemite, putting everything back in place manually.
At first I was starting to have those graphic glitches, and they were going away when doing a fast user switch. I then repaired permission and ACL on the system, and they stopped. I got it working fine for 3 days, without any problem and I though the issue was gone. But still, I didn't feel the system was snappy enough as it should have been after a clean install, especially when it came to graphic effects triggered by gestures (Launchpad and Spaces). Plus many times fans would go for maximum spin despite nothing was intensively using CPU or GPU.
Last night, while performing a DVD burn for backup and other backups to external drives, the computer was having the fans going on really high, but I guessed it was so because of the high traffic with transferring data (Intego Virusbarried also kicks in with behavioural control and real-time scanning when doing file transfers, plus there is also FileVault encryption). Then bum, the Kernel Panic and the rest of the story already mentioned above.
On my computer the Logic Board was changed one year ago, after having had to be at the Genius Bar many times. It costed me around €500,00 on a 2yrs old PRO machine that was out of guarantee just from one month.
From my experience I think there is a problem with the new drivers in Yosemite for these graphic cards.
Next thing I'm gonna try is a clean install of Mavericks and then let it update to Yosemite to check if it'll work. in this way.
Latest note and vent: I spent a freaking all night long plus one morning on this issue, feeling I was on a sluggish Windows PC instead of on a Mac.
What the heck is going on? I'm having lots of negative experiences with this Apple product, and it's a PRO computer. I had to spend money on ti to replace its logic board just after 2 yrs of use. I'm having lots of issues with just switching to the new OS.