Here is the information we have been waiting for!
If you haven't already seen it, pickup the November 2006 issue of HOT ROD magazine. There is a great story (with pictures) of Bumblebee in the making at Saleen in Troy, Michigan.
According to the story, there were two versions of Bumblebee built for filming. The first one was completed in 30 days and both done in less than 45 days. Fabrication was done at Saleen's 280,000 square foot facility in Troy, Michigan. Steve Minn, vehicle coordinator for the film, recommended Saleen because they had built the Cobras used in xXx.
GM looked to their inventory and found the current GTO was the closest to the concept Camaro in size. With data and some sections and wheelbase drawings from GM, Saleen began their fabrication. The two Pontiac GTOs came from GM's R&D fleet and Joe Miesak (Saleen) went to work.
The fiberglass bodies were cast using the same molds GM used to build the two concept cars.
The interior looks much like the concept Camaro because the dashboard shell covers the original GTO instrumentation and the seats and interior handles came directly from the GTO as well.
GM had a say in the paint work. They altered the original stripes planned by Saleen to more closely resemble the Rally stripes from the 1969 Camaro.
Here are some images from the HOT ROD magazine spread: