I wouldn't say common but doesn't effect performance. Just more poor build quality from RIM. Also, look how your trackpad is slightly twisted. The lower right corner is higher than the lower left corner. Yours is good compared to many that are really crooked.
That's the light diffuser inside the keypad. Below standard quality from RIM. I have this on my 9900 and I've seen it on all the 9900s I came across....
Don't worry about it.
I had this on my first 9930 but not my second (replacement) one. Like others have said, it doesn't affect performance, but I know what you're talking about.
Try turning your screen brightness down to 10% to see if that helps. Just a thought.
No matter what backlight setting I go for, I do not have this problem. I have the light evenly distributed. I think this is an issue with poor quality control.
My first 9900 was an unlocked Telus on at&t and it had even lighting but very noticeable crooked touch pad. My current 9900 is at&t branded, mexico 4 build as well, and it has even light distribution and an ever so slightly crooked touch pad. Keys are a little squeeky though.