According to the blog post from former Radical staffer Christopher Scott Hassell, who is now a lead environmental artist at LucasArts, after finishing up Prototype the team moved on to Project Treadstone.
The "action-adventure," spy game had "cover based gunplay, vehicles and parkour style locomotion," and Hassell posts a few bits of art he helped create for the game on his blog.
Hassell said the project was cancelled due to the Activision Vivendi merger and the firm's statement that it already had a the James Bond franchise to fill the spy genre. He also said the firm cited "several other decision points," for the cancellation, so he and the art team were moved over to the Scarface 2 project, which was also cancelled.
You can check out the screens he posted from the game over on his blog, and see a rough demo of the Bourne game in action below.