Update: Some readers are experiencing issues with the redemption codes and are being told by Apple support to retry them later today. For those trying to get codes now, it could take up to 72 hours to receive them.
For those who qualify, Apple's up-to-date program for OS X Mountain Lion is now live. Those who purchased a qualifying new computer or refurbished machine from June 11 through July 25 have until August 24 to file for a redemption code that can be used in the Mac App Store. Computers purchased after today have 30 days to file for a redemption code if Mountain Lion wasn't already installed on your machine.
To complete the form, you will need to know the day you purchased the machine and the serial number. Fill out the forms, the code will be emailed to you, and then you can enjoy your new copy of Mountain Lion!