Fcron is a proper implementation of cron - i.e. it puts itself to sleep when not in use. I've not done any scientific testing, but if it actually switches modes I see three bars on the CPU meter in the staus area for about a second, if it doesn't switch, I see a single bar for about a second. I figure that will be a lot less power use than having 3G, but again, nothing to back that up, other than the fact that I don't have to charge my N900 more than once a day now.

If you think it is too much, you could change the cron job to run every 5 minutes instead of every minute. To do that, use

- although be aware I haven't tested that myself.