@Vache, i've read your twitter too late :-(
Yesterday i've tried too unlock the bootloader with Unlock_firmware.zip, reboot and... nothing, black screen.
You know why...
But the S500 isn't dead, when i connect it to USB port, it's detected as (on linux):
I think we can download a firmware with a special tool from Acer ? But do you have one or an other solution ? i bet no...
I believe that i need to return my S500 to an Acer service repair :-(