I hope they come out with a fix, unfortunatly I didn't have a drive to be able to use Time Machine so I could not make a backup.
I got both Lion and Mountain Lion from the App store, I bought my Mac last Sept after Lion had just been out for a bit so I was eligible for a upgrade... although the store said macs ready with Lion mine was not... but no biggie I updated it, thought I did find it odd that in Sept when I got it, it was still a Early 2011 model I would have figured it be late so no Power Nap for me... Anyways back on topic, I had a $100 credit to the app store I got for getting my mac at my schools Apple store so I thought eh, why not. I still had most of the credit so in a way Mountain Lion was free... I read as many reviews and opinions before doing so and I did not see anything regarding flicker issues until I saw it myself and searched specifically for that.
I do hope they update or something to fix this, its very frustrating. I can't imagine trying to back everything up manually elsewhere and do a clean install.
I tried to reset PRAM but it will not reset, cause I don't here the two start up tones your supposed to hear, I wonder if the settings or steps changed for Mountain Lion.