I use analogReference(INTERNAL); in the setup. The ATTiny85 also has a 1.1V internal reference voltage, so this should work there. Works very well on the Arduino! Before using this, I was getting lots of stray readings (even though I was running off a steady 5v USB supply), and as soon as I switched it, I am now getting very steady readings. I hope those steady readings will translate well to the use of batteries.
Which brings up another point. What do I need to do about low batteries? I saw something that said the ATTiny can be damaged if the batteries get low, and you can use the built-in BOD, but I never saw a good example on exactly how to implement it. DO I really need to worry about that?
If I get enough life out of the 2 AA's, I would be interested in moving to maybe 2 CR2032's in parallel. It would make a smaller package.