I only run it when plugged in to the charger or to a USB socket. But I'm one of those who has never been able to get a full day out of the N900 battery if I leave WiFi on.

Power consumption is very heavy for the first six hours while Bitcoin is downloading the cryptographically-signed block chain. You absolutely can't do that bit on battery power, nor can you do coin generation on battery power.

If you're just doing transactions, the data "chatter" is fairly light and I don't think it would be a battery life dealbreaker.