Start the recording using the MIDI piano keyboard? Is it possible, in GarageBand, to start/stop the recording using the midi piano keyboard as it is, for instance with Cubase? My piano is quite far from my mac and I want to start recording from my piano. (I know about the count-down feature)

PS : With cubase, you can define a key on your piano that will be used as command key.
If you have any other buttons available on your keyboard, you can use the GarageRemote software to program them as transport controls for GarageBand. Instructions for M-Audio Axiom series keyboards are available here. I'm not sure about programming an actual keyboard key.