I can have the N900 continuously connected to 3G for more than 2 days if there is very little actual data transfer (and I do have mail checking set to every 30 minutes). So either it's heavily network dependent or you have some widget/app transferring data very often.

Skype, SIP and IMs do relatively lot data transfer, so having those connected does not count as connection idling.

And when battery power consumption is relatively low, so is the total electromagnetic radiation output power of the device...