One thing I love about AVR chips is how electrically hardy they are compared to most ARM devices. Most can run at 5 volts, and source tens of milliamps. Most also run well at 3.3 volts, which is especially good when interfacing with an ARM.
One thing to watch out for is that the maximum stable clock speed for an AVR is reduced when running at lower voltages. ATtiny85s cannot run reliably at the internal pll'ed clock speed of 16MHz, when powered at 3.3 volts. I've cried myself to sleep over this, so I offer this cautionary tale. Read your datasheets!