[Xperia Play Q&A, Help & Troubleshooting] mattnmag:
@ ultimatelegend1,

To get root access, you don't need to unlock your bootloader, (unless you want to), and you should not have to downgrade to 2.3.3. If your R800x is on 2.3.4, you can use Eroot version 1.3.4 to gain root. Check out the original post in this thread for details on Eroot: http://xdaforums.com/showthread.php?p=54011429

I have used Eroot v1.3.4 to successfully root 2 different Verizon R800x,'s and it should work for you on the current 2.3.4 build. (Just as a side note.... my friend let me borrow one of his older Xperia Plays with a malfunctioning flex cable.... so I was able to play with it to test out things and learn to Root/ROM the device without ruining his other "daily driver" phone. We both figured that if we ruin the one with the messed up flex cable, then oh well, because he couldn't really use it anyway. We later replaced the flex cable, so now he has 2 functioning Verizon R800x's :) ).

Anyway...the other option of using Flashtool to downgrade to 2.3.3 and then rooting with SuperOneClick's built in Zergrush exploit works good too. However,wiping the data when downgrading seems necessary, because I found that I encountered force closes on the downgraded 2.3.3 build without wiping the data from the previous 2.3.4 build. I didn't want to downgrade my friend's "daily driver" phone....because he wanted to keep his data intact on 2.3.4 and run it stock rooted.

As stated before, with a locked bootloader, you cannot use custom kernels, but there are some custom ROM'S that work on a locked bootloader with the R800x. I believe the original post of this thread discusses that, and you can search the threads to find them too. On the Xperia Play, you can also install Clockwork Mod Recovery with a locked bootloader (which is important whether you unlock the bootloader or not).

For unlocking the bootloader, I tried the testpoint bootloader unlock method on the Verizon R800x with a Windows Vista computer... and got a message that said something like "START NAK: 03000A"..... further research led me to believe that the testpoint method doesn't work for the Verizon R800x. Sony's Official Bootloader Unlock method did work though , and I unlocked my friend's extra Verizon R800x....then we installed Doomkernel and Cola ROM and it's really nice. Be warned that using the Official Bootloader Unlock method wipes your data on the phone, so back everything up first. It also causes you to lose DRM keys and (I think) that DRM keys are needed for Track ID and some other Sony apps and certain purchased media to work... but this didn't really seem to effect anything important on my friend's phone as far as I could tell.

The order I used for Rooting & Unlocking bootloader was:

-Copy and save contents of external SD card to computer for safekeeping

-Root with Eroot v1.3.4

-Used Titanium Backup Pro to back up all apps (copy the backup to the computer for safekeeping)

-Installed Clockwork Mod Recovery and made Nandroid backup (copy the backup to the computer for safekeeping)

-Made a TA backup with the Omnius Program (and copy the backup to the computer for safekeeping)

-Unlocked bootloader

-Finally installed custom kernels/ROM'S.

(And just as a disclaimer to cover my butt.... however you decide to use this information is fully your choice, and you are taking full responsibility for your actions for whatever happens [good or bad] to your computer/phone if you choose to try to modify things).

Good luck! :D