Here is standard Virgin Mobile USA Customer Service reponse to my e-mail:
Yes, the update will start with 5,000 Intercept users and we will continue to be released in waves of 5,000. There are no specific criteria to be selected first for the update other than you have to have an Intercept. The update has been tested so the first group is not to test the update we choosed 5,000 at a time to make sure the systems wont overload and will not crash our systems or skip anyone. I do not have a specific time of when the update will start or be finished by but, my guess is that the update will start around 8:00 AM central time as to not wake anyone with notifications early in the morning and I do not think that it should last more than a day or to for the update to reach everyone. If there is an issue like the famous "Bricking" that some devices have experienced during software upgrades you will either have to deal with the place of purchase or if they will not help we will send you another phone in a timely manor. That should not happen due to extensive testing. If you do not wish to wait until the system notify you to update you phone you will be able to choose the option to manually update. I highly recommend backing up all person information like music, contacts, calendar events, memos, applications, or anything else that is important to you.