Just noticed the new ATtiny43U. It can run on a power source in the normalish range (1.8V-5.5V), or from one as low as 0.7V using an on-chip boost converter, perfect for a single alkaline, NiCad, or NiMH cell. It will produce 60mA at 3V, so you can use it to power a couple of other low-current devices as well.
Apparently this has been in the works for a while, as WinAVR already supports it (nice!). I was more excited until I looked at the datasheet and saw that you had to supply external components to use it (inductor, diode, capacitors). I also wish it had a real USART and not just a USI, but its still pretty darn cool!