Sticking your chopsticks upright in a bowl of rice is a no-no because that is
the way rice is offered to the dead (not necessarilly because it looks like incense sticks). This is called 'hotoke-bashi' in Japan - hotoke means Buddha (the bashi or hashi part is chopsticks), and a deceased person is considered to have become one with Buddha. But spearing food with chopsticks for the purpose of easily picking it up or so is just a etiquette no-no - rather like you're not supposed to spear food with the tip of your knife and carry it to your mouth in European cultures. (At least, not anymore. Before the invention of the fork that's how people used to eat.) In Japanese it's called sashi-bashi (spearing chopsticks).
ETA: As I said to Garrick in the comments, the reason why spearing food with your chopsticks is considered to be rude is partly because it's regarded as looking sloppy, and also because it reflects poorly on the preparer of the meal since the food presented to you is supposed to be easily eaten with ones chopsticks.
Incidentally, the rice bowl photo that User used* comes from one of my articles about chopstick etiquette with many other faux pas - take a look at the whole thing if you're interested, including the comments from people from other chopstick cultures. http://www.justhungry.com
(*I should really change that photo - it's not a good representation of hotoke-bashi. A rice offering would be a perfectly molded bowl of snow white rice (the 'best' rice) with the chopsticks standing up straight in it. I juse used that bowl of mixed-grain rice because that's what I was eating at the time ^_^;)