This has been happening to me recently with my MBP as well, but only when plugged in to an older power cord from the old white MacBook. It does not happen on battery, and does not happen on the power cord that came with this MBP. The keyboard does not light, and hitting either the brighter (F6) or dimmer (F5) button makes the keyboard backlight icon appear on the screen but it's greyed out with a circle with line through it symbol under the icon for the backlight and above the marks where it normally indicates how intense the brightness happens
I have in the past had a similar problem where the internal speakers would not work, hitting any of the speaker buttons would show the speaker icon on the screen, greyed out, with a circle with a line through it between the speaker icon and the marks showing how loud it is. In addition, when I went to the System Preferences while this was going on and I looked at Speaker, it said I did not have an internal speaker installed. What fixed this temporarily was flashing the PRAM, but it would come back again at random times.
The keyboard light problem always fixes itself though I haven't figured out when yet (maybe when I unplug the old power cord? maybe with time?) so I haven't needed to try flashing the PRAM. Next time though I am going to check in System Preferences to see if it thinks I don't have a keyboard installed or something. (I mean, the keyboard *does* keep working, but we'll see.)