Regulating AA to 5V and achieving high efficiency
Although there are several decent DC-DC Boost Converters available, I suggest that you try to use 2 "AA" cells if you have the room.

I currently use the LM2623 boost converter from TI (formerly National Semiconductor) in several products.  This is a pretty darn decent device that will work from a single "AA" cell.

In a nutshell, the LM2623 runs from 0.8V through 14V and has an on-board switch rated at 2.85 Amps.  The input voltage can't exceed the desired output voltage.

The TI product page is LM2623

I'm also using the Microchip MCP1640 boost converter.  This doesn't have as much current capability as the LM2623 but has the decided advantage of disconnecting the output load when the chip is shut down.  This was very important in a couple of my projects.

In a nutshell, the MPC1640 starts with an input voltage greater than 0.65V but will remain running down to 0.35V.  The internal switch has considerably less current rating than the LM2623 at 0.8 Amps.  

Be aware that the MCP1640 is available in 4 different variants.  We use the 1640 &amp; 1640D versions.

The Microchip product page is MCP1640

You should also have a look at the Adafruit MintyBoost project page.  This uses a different boost converter and runs from a pair of "AA" cells.  The project is useful in its own right but even if you decide to use a different boost converter, the USB interface information is invaluable.

The Adafruit MintyBoost project page is Minty Boost