This might something as simple as the USB device initialization. Sometimes USB plugs don't start talking to their connected devices until after the boot splash screen. Switching your keyboard to a different port, especially one on the motherboard itself (as opposed to a case port or USB hub), solves the problem. After the update, your USB port just got bumped up the initialization list.
Intel i5-3570K / Asus P8Z77-V (1805 Bios)
Samsung 1.35V 1600mhz DDR3 2x4GB / Samsung 840 128GB Boot Drive
WD Green EZRX 3TB media drive / CM Hyper212+ AS5
HD4000 iGPU / SeaSonic Fanless 460Watt Gold
Fractal Designs Arc Midi / LG BluRay burner / Win7 Home 64bit