I've just realised that the radio transmitter has an 11mA (max 22mA) specification. Since my Duemilanove can supply 40mA per digital IO pin, I can use the digital IO to power the transmitter with a simple connection. This means that I can turn the transmitter on just before sending the signal (20mS min) and off afterwards. The transmitter won't be broadcasting the carrier wave all the time, and the power consumption is reduced.