For the original question, at idle the consumption for 2g/3g should be similar as long as you have stable coverage and are literally at idle, which with a device like this is difficult unless as was noted you are asleep !

I have very good 3 or 3.5g coverage at work in the uk and can leave the handset at idle for 8 hours solid and it uses very marginal power at all .

the overnight test results would be very interesting to me

James