I think this is a great project! But Raspberry Pi might be a bit overpriced/overpowered for this task. Maybe something like Arduino Yún would be more appropriate choice. I am really hoping this movement of small GNU/Linux based home appliances will take off and lower the price.
How would using a Yun be cheaper than a Raspberry Pi? The Yun is currently twice the cost of a the RPi, and even taking into account the WiFi adapter and SD card, the RPi comes out far ahead.
You would also need to take the time to write drivers for the USB microphone and do realtime speech recognition, web traffic, and text-to-speech on the Yun's little 16MHz/400MHz processors.
This project is perfect for single-board computers like the RPi, and I can't imagine that you would get that far with a microcontroller.
