I've had to investigate a similar issue on a friend's MacBook Air (13 Inch, Mid 2013). The keyboard and trackpad both stopped working, although they were able to work pre-boot (as in, you could use them for keyboard commands to get into recovery or verbose modes, NVRAM/SMC reset and for use in the boot selection and diagnostics screens). However, a fully new install of OS X Yosemite hasn't fixed anything, and neither have a NVRAM or an SMC reset. When booting into single mode you do see a vast number of error messages, which also crop up in the standard error console when using the machine with a USB keyboard and mouse.
24/04/2015 16:30:30.000 kernel[0]: AppleIntelLpssGspi1::initFixedDMA: FixedDMA not found in _CRS
24/04/2015 16:30:30.000 kernel[0]: AppleHSSPIController::start Start Succeeded
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: AppleHSSPIController::_enumerateDeviceOnThreadGated Already attempting to enumerate device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIDevice::_DoDescriptorRequestGated Failed issue request error: 0xe00002eb
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIDevice::getDeviceDescriptor Failed to get descriptor with error 0xe00002eb
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
24/04/2015 16:30:30.000 kernel[0]: Error: AppleHSSPIController::doSPITransfer NAK received from device
... and so on
The System Profiler also says that no SPI devices were detected. The complete reinstall of OS X did make the keyboard backlight activate, but nothing else has changed. Bluetooth status makes no difference either. It appears the device is genuinely dead.