I have had the same issue - same phone. Noticed it first week, then it's slowed down in occurance but then I'm also not having to input my password as much. It's been exclusively found in the AppStore, which makes me also think it's been an OS/software issue.
I was able to take a photo of it using my girlfriend's Droid...which has never had a screen flicker issue.
There they are, right over the - / : ; and ending before the ( starts. This has been consistent to when it happens - it happens there every time. It seems to get worse and then not happen again...but occurance, dependent on me entering my password for any Apple software program, has become more infrequent as I'm not stocking up my apps anymore.
I'd love to see any other photos people have managed to take...