I can verify this works. I just did this (thanks to the recent posts by markwj in this thread).
I had already had Feature #15 set to 1 (so I could do things like lock/unlock the doors). I just sent the "Feature 13 1" command (via SMS). I received "Feature has been set" back from the car and, lo and behold, I now get my speed on the little amber display above the steering wheel! The only negative I see to this little hack is that it still says "amps" next to the number. I very minor complaint, I know.
A good test to see if it works is to sit in the car with the ignition on, aircon on, but car in park. You should see about 1 to 3 Amps shown on that display.
Then, SMS "FEATURE 13 1", and the amps should go to zero. After all, you're going 0mph.
The feature is far from perfect, but pretty usable. Nothing we can do about the word 'Amps'. You may find the display flickers every now and again (changing to a weird value, not your speed) - again nothing we can do about that as once in a while the car will win and display the real Amps (as opposed to our faked speed). Watch the video for more information on why that is.