I wrote to their Customer Support folks and received this response:
Dear DivX User,
We apologize for any delay in receiving your FREE serial number. Our mail servers have been working like CRAZY this past 24 hours to handle all of you guys
Don't worry, you will get your email with a serial number, but it is just taking a little bit longer. It could take up to 2 days before your receive your serial number. So please be patient and give us until beginning next week before you come to our Support Team for a serial number
Enjoy!
DivX Team