Do you have a case on it? Sounds stupid but I had this problem with the OEM soft shell. I would push the sleep/wake button, and nothing would happen. So then I would mash on it like 50 times then the screen would respond like 20 seconds later.
Sometimes it happens to me due to the pattern lock application I am using. To be precise: pattern lock would be locking the phone after it has been inactive for the set number of time, and in the same time I would try to wake the phone up. Then it seems not to respond for maybe up to 15 seconds.