[blockquote]Roberto Vengut Tro said:
GPS tracker brick (also usefull for speed measuring)
[/blockquote]
I don't think GPS is useful unless you plan to travel great distances outside with your device. GPS can't supply enough detail for speed measuring on a Lego-scale.
I really like the video streaming idea.
Yes, the two outer wires are + and -, and are NOT used by the PF motors. Only the servo motor uses them to get the constant power it needs for it's electronics.
Also, the IR receiver and the SBrick too draws power from these lines.
The two middle wires are the control wires, they are used by the PF motors. The servo motor listens to these lines to determine the angle it should turn it's axis.
SBrick can also draw power from the control lines, so it can be powered by tran / old power supplies.
A lot of detailed information can be found here: http://www.philohome.com/pf/pf.htm
Also, I was wondering if you could use the other two lines for this. The PF cable has 4 wires, two for electrical power, and two which I don't know exactly what they do. Maybe they can be used for input, perhaps LEGO has plans for i2c also on the PF series.
In this case, maybe a project for a whole servo motor with position feedback would be also nice!
A rotary encoder device (similar to MS GlideWheel http://www.mindsensors.com/index.php?module=pagemaster&PAGE_user_op=view_page&PAGE_id=173) would be really nice.
That would allow to turn PF motors into servos (but without the limitation of the LEGO PF Servo of 180 degrees range)
GPS tracker brick (also usefull for speed measuring) , programable 8 RGB LED light brick, rpm measure brick (also usefull for return to center and other robotics things because it will count exact rotations), touch sensor, G-force measurer brick.
Yeah, I have too much imagination :P