Very cool. Are you familiar with Seed Studio's PCB service? I think they're about the cheapest we've found for 2 layer projects. I don't think they do four layers. Also, I love that you're using the ATTiny. That little chip is so cool.

I've been eyeing the ATTiny25 (eight pins) for a couple of years as a sensor controller/communications unit. Hook the ATTiny25 up to any kind of sensor. Use two pins on the ATTiny as an I2C bus client, and you can put a network of sensors controllers all over the place on things like aquariums, or model rockets, etc.