It is not very clear from the data sheet ( http://www.atmel.com/images/Atmel-8271-8-bit-AVR-Microcontroller-ATmega48A-48PA-88A-88PA-168A-168PA-328-328P_datasheet_Complete.pdf ) that I looked at either. Near the top, they talk about the frequency range and mention the "speed grade" 0 - 4MHz@1.8 - 5.5V, 0 - 10MHz@2.7 - 5.5.V, 0 - 20MHz @ 4.5 - 5.5V which seems to indicate it is a static device and can go all the way down to 0 on the frequency, however, in looking at the area around pages 29 and 30 in the data sheet above, it seems to be more along the lines of 400khz as the lower end of what they are showing. You might try contacting support at Atmel and asking them - some processors I have worked with in the past are "dynamic" and have a minimum frequency to be able to "remember" what they are doing while others actually are static and can go all the way to 0 on the clock without "forgetting" what they were doing. You may also find a way to do what you want in looking at the sleep modes since they affect what the clock is doing.
mikey