Smallest CPU supported by Arduino IDE

I am not sure if I am making the proper question, so I will explain my goal first.

I would like to use a second CPU/MCU for an existing project involving ATmega328P (customized hardware). This new CPU will only do 2 tasks when not sleeping: read one I/O pin and depending on the value write or not at other I/O pin. Just this, nothing else (except sleeping).

Based on this description, any CPU seems to satisfy my needs. Therefore, I am looking for one that consumes the very tiny amount of power during sleep/active cycles. At the same time, I'd like to use the Arduino IDE, libraries, and so on. What is the best option for the CPU given these requirements? I am not sure if I am constrained by only the options at the menu Tools/Boards (ATmega328, ATmega1280, 2560).

Thanks for the attention.

in PDIP package i think atinny85(or 45) is the best choice.
and heres how to set it up: http://hlt.media.mit.edu/?p=1229

more info: ToasterBotics: Using Arduino with ATTiny's
ToasterBotics: Programming the ATTiny85, Arduino Style

Other places....

Support files and directions to use ATTiny-45/85 from programming to stand-alone are here:
http://hlt.media.mit.edu/?p=1229

Youtube on the subject:

Mouser has ATTiny's for less than $1.50 ea.

ionito:
This new CPU will only do 2 tasks when not sleeping: read one I/O pin and depending on the value write or not at other I/O pin. Just this, nothing else (except sleeping).

Do you actually need an Arduino to solve this problem? It sounds like the sort of problem that could probably be solved by a transistor.

Thank you very much for the link "Using Arduino with ATTiny's ". I will read and return my experience on that.

PeterH, actually I started from your viewpoint also. However, the sleeping function of the MCU cannot be achieved using discrete components, such as transistors and comparators, with the same or smaller microwatt consumption level. I made a research about the options and the best option I found (low-power timer) would consume more than 5 times compared to ATmega328P.

Thanks for the contributions and insights.