I've just got my Tama-Go today! :D
I noticed that on the game start screen, the top several (around 5) rows of pixels on the screen (upper part of the "READY" text) are darker than the rest of the pixels on the screen. Here is a picture of it:

For people who have Tama-Gos, could you check if it happens to yours as well when you have a chance? Thank you very much. :)