Apple already uses alphanumeric serial numbers, but it has long been possible to determine the date and location that a product was manufactured based on the current format. Readers would often use serial numbers to glean more information about their devices. The randomized format would likely not be decipherable, or at least hard to, and it could also help to reduce fraud.
The memo was published today and obtained by MacRumors from a reliable source. It is unclear if the change will apply worldwide.