For those who are familiar with the E71, it probably has the best battery life on the market. Uses 1500mah BP-4L.

I ran some tests to compare the E71 and n900 battery usage, both pretty much idle, almost no screen usage, few mins phone calls.

Results:
E71 = 60 hours to get to 30% left
N900 = 24 hours to get to 30% left

Since both are idle and no screen usage, the n900 less battery life is not because of screen or running apps. Either the GSM radio is more power hungry or Maemo needs more optimizations in its background processes or idle state. Or what "black magic" is the E71 doing?

Test case:
- N900 completely stock. No apps ever installed. Has gone through 4-5 recharge cycles before this test
- no 3G. GSM only
- no WIFI whatsoever
- no Widgets
- screen on lowest setting on both units
- N900 automatically connect to At&T internet 10 min interval
- Both running Nokia Messaging with gmail,yahoo,hotmail every 30 mins.
- Both constantly connected to gtalk+skype (E71 uses Fring). So the internet is constantly connected on both devices.
- max 5 mins screen usage (only to check battery + take 5 mins of calls)
- each day involves 40 mins underground with no reception

E71
-----
day 1: 09:30 -> 09:30 => 70% free
day 2: 09:30 -> 09:30 => 50% free
day 3: 09:30 -> 21:00 =>30% free

N900 (repeated 3 different days, same usage, same results)
------
day 1: 09:30 -> 09:30 => 28% free

Outside these tests, I notice if I use the device ontop of the above usage with (again with no 3G)
- 30-40 mins web surfing
- 30 mins bluetooth music
- 10 mins misc playing around with the device
The battery is more like 09:30 -> 00:00 => 25% free
With the current battery life, I'd be very wary of moving to Tmobile and using this device on 3G and lasting 09:30 to 00:00