As reported here by BlackberryClub Forums moderator Mark Rejhon, Nexttel models are the first to recieve Blackberry OS 4.0. Other models will be soon to follow.
For more information on availability, and what advantages OS 4.0 has over previous versions view the following link:
http://www.blackberryforums.com/viewtopic.php?t=2648
And again big thanks to Mark Rejhon for finding this information!