guys, what will happen if I leave my phone for extended period of time say couple of months. without charging. will i be able to charge my battery again (as it will be dead by that time) from my original N900 wall charger? or do i need to buy external charger to revive it.??

Charge it fully before disposing it and you should be fine. If it gets completely discharged it might not start charging right away but it should in a couple of minutes. And take it out from your phone and don't leave it in a place with high humidity.

if it's steady orange when you plug it in again, leave charging without botting up untill it stops being steady, then leave it like about half an hour pulsing and only then try booting it up again

charge the battery to 60%, NOT full, and remove it from the device.

charging it fully and storing is not ideal for the battery... That's why they comew halfway charged from the manufacturer.

Dantonic's advice is good and incidently applies to laptop batteries as well....

yes it applies to any li-ion battery, if they hit 0% they are damaged and will not recharge. Charging it to 100% and letting it discharge over an extended time also reduces the capacity.
Anything around 50% is ideal, and also storing it at less than 15 degrees C helps.

http://www.buchmann.ca/Chap15-page2.asp

It's actually ideal to have it at 40%, or just a little less.

Store it at a low temperature as well, and your battery will love you.

the protection circuit of the battery uses about 3% per month, if he stores it for a year there goes the whole 36% charge, maybe more if it was not kept cool... so yeah 40% is okay but after a year maybe not a good idea.
got that from here: http://www.buchmann.ca/chap2-page2.asp

Does it really get bricked if allowed to discharge completly? Even if you plug it in the wall charger?

yes, according to that book i was quoting it says they are dangerous to charge at that point so the protection circuit purposely bricks it