I'm having the same issue - when I come back from viewing photos to the menus, the backlight briefly fades out and then comes back on. I had always thought that w3rd's explanation was responsible - because viewing photos uses different brightness settings, it had to power down the backlight before it could load the new settings. Regardless, it's not much of a problem even if it is a bug.
I think it's because of the different brightness of video and "normal mode".
Try to set the brightness in main menu - settings - display settings to 100.
I've got the same issue, I don't think it's hardware-related because when playing with the brightness settings, the backlight doesn't seem to need to be shut off before being set to a new (lower) brightness level. Maybe this will be corrected in the future...
i've also noticed this but since it doesn't really affect my use of the miniplayer, i don't really mind it. i just consider it as a transitional effect when exiting photos.