Many hybrids and BEVs have a feature called "Regenerative Braking". This was Recently discussed on the Tesla Motors Blog.
It appears that the Roadster may activate regen when you let up on the accelerator pedal, but may not add any extra regen based on brake pedal pressure.
(In other words as soon as you start to put pressure on the brake pedal you start to engage the friction brakes).
Apparently many other BEVs did tie the regen to the brake pedal, so it would be interesting to understand why Tesla chose not to do this.
Also, people have been asking if Tesla could add variable regen controls to their cars... Apparently the regen profile changes depending on 1st gear or 2nd gear selection, but otherwise is not intended to be driver adjustable.
Here are some examples of places that mention brake pedal tied regen on other BEVs:
In an account of driving an EV1, John wrote:
"..time to employ my usual high miles per charge techniques. I switched off the coast-down (we EVers call this off-throttle regen) features, so that I could let the EV1 glide along on slight down hill runs without the car trying to slow down putting a small bit of juice back into the batteries (I'd save this feature for later, as when exiting the freeway and for congested traffic stop and go driving)..."
If you go to the EV1 FAQ, and go down to the bottom there is a section called "What is regenerative braking". They mention " 'Regen' is the process of making the EV1's motor work like a generator when the brake pedal is pushed, or when the vehicle is coasting with the 'coast regen' on."
First Peek Inside the (AC Propulsion) eBox:
"When I applied the brake pedal, the regenerative braking was smooth. When I pushed the pedal further, the transition from regenerative to mechanical braking was seamless."