Some of my friends pour cold water on pasta after boiling it other don't. What do the professionals do? If they pour cold water on the pasta after the boil, what do they want to accomplish with doing it?
I would strongly suggest not pouring cold or any other kind of water on pasta; it will rinse the delicious starches off its surface, which would otherwise help your sauce cling to it.
To counteract the negative consequences of overcooking due to remaining heat (which, as belisarius suggests in his answer, is the reason other stuff is often rinsed with cold water), just serve it immediately. You want to have the sauce (hot and) ready just before your pasta is, so you can drain the pasta, mix with the sauce, and put it on the table.
Pour cold water on boiled things when you want to stop the cooking process. Once you get "al dente" stage, the pasta will quickly get past that, so cooling it with cold water helps you manage the final state.
Be aware that the pasta will lose temp, that is going to be re-gained when you briskly sautee it afterward (with some salsa, hopefully).
PS: for vegetables, quickly immersing them in ice-cold water after boiling helps to retain the bright color (although I don't know why).
Your pour cold water over pasta if you don't want the noodles to stick together later. If you are making enough for the amount of servings -- that you will eat right away -- then it is ok not to. As Erik's answer pointed out, the sauce will stick better onto the noodles.
So you are cooking pasta in bulk and will refrigerate some of it for later consumption, it is better to dump cold water on it. Otherwise, you'll end up with a glob of pasta that you can't sort out later.
Even though you might want to rinse pasta that you plan to serve warm with a sauce later, to do so is never ideal. Rinsing in cold water then tossing with a bit of oil works adequately to keep the pasta from sticking if you need to serve the pasta long past the time that it is cooked, but even under those circumstances, consider tossing with the sauce in advance instead of rinsing.
If you're making a cold pasta salad, then rinsing with cold water might very well be your best, ideal choice. It depends upon the dressing, but that starch that makes the pasta stick so nicely to warm sauces can make cold salads unappealingly clumpy.
Alternatively, I use a little bit of oil to coat the noodles and then stir it up. This way I can prepare or finish off whatever I am doing before I sauce the noodles. This method is also ideal if you are making an intricate noodle dish like ramen (real ramen, not instant) or pho where ingredients are layered or designed in specific ways that can take some time. You then just pour the very hot soup base and the noodles will instantly loosen up. i also use this method if I made too much pho and we need to save it. Oiled noodles won't stick, will loosen in liquids and can be refrigerated.
Erik P. is absolutely right, rinsing pasta is a bad idea, and many Italians consider it heresy. Rinsing removes the residual cooking water, which contains considerable flavor, nutrition and texture. This water is often used in recipes as the primary base for sauces. Rinsing also lowers the temp of the pasta. This will stop cooking in the pasta, but you will have to add heat to bring it back up to temp, which will lead to further cooking. Really, pasta is pretty heat tolerant, and the cooking time for al dente pasta and overcooked pasta are not that close to each other. To prevent the pasta clumping, add salt and oil to the cooking water before the initial boil. If you are storing the cooked pasta sauceless, toss it with a little olive oil. This is how it is done in restaurants.
Yeah, it is better to wash the pasta with cold water (to wash away unnecessary starch that sticks outside) very fast... otherwise the pasta gets cold. After that, you can mix the pasta with the sauce and bring it to boil for about 30 seconds. If you don't wash it the pasta will stick together and will be hard to eat later. If you wash it you still can eat it even after 2 to 3 hours.