OK, here's how my N900 fared today, with a standard 1320mAh battery:

things always on:
screen at 40% brightness
connected to WLAN, if available
connected to GSM/GPRS/3G where available
photo changing widget (every 5 minutes)
weather updated on the hour
RSS updated on every half hour
Mail for Exchange always in online mode
one IMAP account always active (different from MfE one)
application updates always on
no screen vibration, no screen or system sounds

6am:
off the charger, battery 100%
checked for application updates
installed 2-3 applications
mucked around a bit, dismissing e-mail alerts
read an ebook for around quarter of an hour

8am:
disconnect from WLAN, connect to 3G
during the journey to work a LOT of reselections between 2G/3G, etc
connect half a dozen IMs, with GPS on as well
Bluetooth on, connect stereo headset
Start music playback
until next time check below did the following:
music on all the time
watched a short film (~10minutes)
read and replied to a couple of e-mails
read an ebook for around 10 minutes or so
tracked a bus journey using Maep for around 15 minutes

9.15am:
stop music playback
disconnect IMs and GPS
Bluetooth off
connect to office WLAN, disconnect from 3G
check for application updates
install 2 applications, remove 1
dismiss half a dozen e-mail notifications
a 5 minute phone call
muck around with settings
spend 15 minutes looking for battery level widget
muck around with battery level widget

11:30am:
battery at 36%

Now, if I continued this usage pattern, I could expect total battery life of at least 8.5 hours. I know this doesn't sound much, but if you look at what was happening (WLAN on most of the time, almost 1.5 hours music/video playback, screen on a lot), that's actually not too bad I think. When my E71 was a new kid on the block and I used to play with it a LOT, the battery never lasted more than a day (usually around 16 hours, with listening to music, WLAN, installing lots of apps, reading/writing mails, etc). These days, when I have N900 to play with, my E71 can last almost a week, as I use it for SMS and an odd phone call.

So, in conclusion, 6h battery life definitely warrants a query with the shop, or an honest appraisal of usage patterns.

Oh, and remember that E71 comes with a larger capacity battery to start with, 1500mAh v 1320mAh in N900, has screen half the size of N900, and runs an OS (S60) that has been optimised for power consumption for a much longer time than Maemo.