using common sense here...
no, indeed, just turning on your WLAN modem or even actually connecting to a WLAN (especially if like on the N900 you can lower the power from 100 mW to 10 mW) does not use that much power.
alas, the moment you get an IP address, the e-mail will get checked (can reduce that power consumption by setting the update interval to 1 hr. though), your favourite Instant Messaging program may go online too, telling everyone you are online, where you are, what you are doing, whom you are talking too, what pretty girl's legs you are staring at and knows gawd what else... )
THAT definitely is going to use a lot'a power
dito any other "client" program updating socal net sites or the like
or, the other way around, RSS feeds
again, if you limit the power to 10 mW (still an option on the N9(50)?) you can limit the consumption, but nonetheless the data has to be transmitted, thus the IP packets sent & received...

now, 2G or 3G networking... considering that the GSM connection isn't as stable as the WLAN one (when you are moving, which is usually the case when you use 2G / 3G) you can't limit the power consumption because if an obstacle is getting between you & the cell, the modem will have to increase the power to stay connected...

power consumption of 2G versus 3G?
think of it as mileage of cars...
if you have to go somewhere on your own, you could go with a small car, thus little consumption...
the whole office coming for the trip, you'll need a larger car, thus higher fuel / power consumption

same thing for your mobile; you can IM perfectly well with 2G because the text sent back & forth need little bandwidth, thus less power.
if you are updating a social site with your GPS position and knows gawd what else, there is going to be a lot more data traffic and you will have to switch to 3G (and use more power).

long story short; any additional device turned on uses power, if it's not used, indeed, the power consumption will be minimal, but it will still drain the battery as well. especially 3G, it is a little like the SUV of comm
and one of the reasons the predecessors of the N900 (770, N8x0) didn't even have a GSM modem; simply used too much power if always connected...