In my opinion "can" and "could" are equally common in colloquial speech. However, "can" tends to replace the more grammatically correct "may" significantly more often.

See Jeff's answer.

So, can I say "could you correct my notebook entry?"? :)

"Could" used in a question is more polite than using "can". "Can" is direct and active. All these depends on who are the speakers. "Can you go into the meeting room at 1pm". This is a direct command from the boss! No ands and or buts. He will not say, "Could you go..." As for telling the Queen of England that you needed to go to the bathroom, you must say, "Your Highness, may I be excused for a moment?" Unless you are a child, you will be considered uncouth. The Queen on the other hand, has the class not to ask for the reason. All she will say, "You may not!" LOL In the shop, you say, "Can I see that?". You will not say, "Could I see that?" (This is wrong if you are asking the salesperson at that time. The past has not occurred yet". To be polite to the salesperson, you say, "May I see that?" If you are an underling, you don't tell the boss, "Can you be at the meeting room at 1pm" but "Could you be...". You don't know or know how sensitive bosses can be!

Both are common here I think its just a matter of what the speaker has become accustomed to using. (Personally I feel I use "can" more often than is grammatically correct.) But if you say "Can I see that?" or "Could I see that?" you would be understood either way, though can is used more often to replace the grammatically correct "may".