Don't be angry. I'm having the exact same problem and I was searching on Google to see if other people were having this problem. I think it's a problem with the game. Also, I JUST ordered a new sensor bar and have been using it for a few weeks because my old one was damaged. It is a sensor bar directly from Nintendo, also.

Notice how when you press the home button, you can go into that far left corner, but ONLY in SMG it can't seem to pick up a signal...(perhaps it's a error in-game where that part of the screen was considered cut-off for the star cursor to go). So, yeah, I basically think it's the game. It is really annoying though, because once I died because I didn't grab a blue star thingie to drag me into a bubble because it was too off-screen. :/

Also, the cut-off seems to be going in an upward slant like so: / (not so dramatic, but you get the idea)

Edit:: I emailed Nintendo saying this:

"Hello, I've just bought Super Mario Galaxy and there is a strange sensor bar problem within the game. I just bought a new sensor bar from Nintendo.com, so I know there is nothing wrong with the sensor bar itself. The problem is, when I play Super Mario Galaxy, there is a strange cut-off in the top left corner of the screen. In that corner, if the cursor is pointed there, it fails to recognize it and the cursor disappears. This isn't a sensor bar problem, or a Wii remote problem, as others are having this problem as well. This can be frustrating at times and I was wondering if it will be corrected in a firmware update or something else. Also, when I go to the Wii menu or the Home menu (while playing Galaxy) the cursor can move anywhere I point to, but when I return to play Galaxy it has the problem. Thanks."

I'll let you know what they say back.