A couple of months ago, rumours and speculations started to surface about a revised version of the iPhone 4 being released which fixes the notorious dropped call antenna issue. A Mexican carrier Telcel seemed to suggest that this was the case and that the fix was due to coincide with the end of the free bumper program (September 30th).
We called Apple’s tech support and according to them, this issue has been fixed in all iPhones manufactured from the beginning of October (when the free bumper program ended). Of course, Apple did not publicly announce this, else that would lead to an onslaught of people wanted to switch their pre-October iPhones to the fixed model. Wearing a bandaid around your iPhone doesn’t take the place of a fixed working device. Seems like Apple is just letting this update slip under the radar quietly.
You can tell when your iPhone was manufactured by checking the serial number: The first 2 digits are the factory number, the third is the last digit of the year (currently 0) and the fourth and fifth is the week of the year that it was manufactured. So a number of 39 or greater would indicate that it was manufactured in October, which supposedly means that you have a revised model.