Q: any options to "untrust" the computer?

Not that I'm aware of. According to Apple:

"If you select Trust, the computer will be trusted indefinitely."

My iphone5 asks the Trust/Untrust question each time I connect it to the same computer.  So it appears that it only retains that setting for individual sessions/connections. 

Not according to Apple:

http://support.apple.com/kb/HT5868

hello

no

that info, apparently confirmed by the ht5868, is wrong.

with some pcs, luckly, that prompt still there any connection

for sure, some configuration options about that prompt are needed in the next ios release

I tapped the Trust option, since it was my computer I was connecting to.

I second that-- I accidentally hit "trust" on my work computer (which I never want to trust), and can't find any way to untrust it.

I'd like an option to "never trust this computer" so I don't need to keep seeing the prompt, and to manage trusted computers going forward.

I submitted this request under rdar://15372764 on bugreport.apple.com as a security issue.

Absolutely a security issue.  It is too easy to click "trust" accidentally when the popup asks every time you connect.  If you do, it is irreversible. 

I deleted /var/db/lockdown as stated in http://support.apple.com/kb/TS5335

This did work for me. All the authorizations are gone! You may need to pinpoint the correct file if you want to delete only one authorization.

What if you want to "trust this computer" AFTER you've hit "dont trust"?

I accidently hit "dont trust" when i first plugged my deivce into the computer. Ive uninstalled itunes and it still wont give me the propmt.

Will deleting the lockdown folder solve this issues for me?

I am afraid you will have to try it, if you want to know.

You could just rename the folder, so that you can try and still don't loose anything.

This solution worked for me ( Unplug iDevices -> Close iTunes -> delete file -> plug iDevice back in )

Also if you're looking to only delete one device's authorization instead of all of them, then just match the file creation times against the day you "trusted".