hello again,
so I think I resolved my n900 battery problem. Here is the summary
1) I noticed battery would run out very quickly (<8hrs) when I had 3g turned on.
2) I tried disconnecting from Internet and use it just as a phone. Battery life was superb..2-3 days and battery was still above 50%
3) Obviously, n900 is not supposed to be used just as a phone, so I installed tcpdump(packet capture utility) on my phone. Yesterday i turned on 3g again and my running programs were skype, googletalk, SIP phone , gmail, exchange mail, omweather. I don't think i have anything else. The battery ran out as usual in 8 hrs.
I examined all packets transferred yesterday with the help of tcpdump (exported to wireshark on my pc) and found out that skype & gtalk were very chatty programs. They were constantly sending out packets which meant that 3g was in constant use. Until today I had assumed that simly keeping the Internet connection turned on via 3g was the reason for battery running out.
4) Today I have 3g connected full time but I shutdown skype & gtalk, its been nearly 8 hrs and battery level is 75%. note: the battery indicator is still nearly at 90% but we now know thats not true...

conclusion:::
BATTERY DOESNOT RUN OUT BECAUSE 3G OR 2G IS TURNED ON, BUT BECAUSE PROGRAMS LIKE SKYPE AND GTALK USE IT CONTINOUSLY TO SEND OUT KEEPALIVE MESSAGES...NOTE: I DIDNT USE SKYPE OR GTALK YESTERDAY, THE PACKETS I SAW WERE JUST CONSTANT KEEPALIVE MESSAGE...CONTINOUSLY EATING MY BATTERY JUST TO TELL GTALK AND SKYPE THAT IM ONLINE
5) next steps. now I need to find out how to shut up skype & gtalk while still remaining online. the skype protocol has been configured so, I believe, to get around firewalls, which is basically why many service providers find it hard to block skype.