[quote author=Coding Badly link=topic=128048.msg963356#msg963356 date=1350681198]
If conserving energy is important, lower frequencies are better.[/quote]... unless you're spending most of your time sleeping. I suspect that running at a higher clock speed can allow you to complete your operations more quickly and get back to sleeping sooner, which may result in a net savings. I admit I have no hard data though.
Fredjikrang, see: Arduino Forum
You'll need to use the fuse calculator linked there to set the BOD to 1.8V (or off). If you want to run at 1MHz, you'll also need to set the internal RC oscillator CKDIV8. Otherwise the above writeup should guide you in the right direction.
And while we're on the topic of clock speeds and fuses, I have a question about the three SUT (Start Up Time) options: 6CK + 0ms, 6CK + 4.1ms, and 6CK + 65ms. What does "6 CK" mean and is it safe to use a +4.1ms setting with an 8MHz internal RC Oscillator? Seems like 65ms is an awfully long time to start up, which would waste a lot of battery power... but I guess it's not much of a savings if the +4.1ms setting introduced some kind of instability.