jailbreaking and installing backgrounder will drain your battery. theres no doubt about that.
i would assume that jailbreaking itself kills battery life. i wouldn't trust those that say their battery lifes are not affected at all. how would they know? those same people probably have not used a non-jb'ed ipad since the day they got it.
for those that have jb'ed. reformat and don't jb. you will notice a BIG difference in battery life.
The act of jailbreaking has no effect on battery life. Simply jailbreaking doesn't do anything else to the device except allow the execution of non-Apple approved code. If you jailbreak and install nothing else, absolutely nothing else changes on your device. I don't understand why people continue to say otherwise, it's simply misinformation.
Yes, running multiple applications in the background effects battery life. My laptop's battery lasts longer if I simply leave it sitting there as opposed to checking email and browsing the web or watching flash videos or even playing a game. I don't think anyone would argue running background apps has no effect on battery life. However I think the slight decrease in my average use is more than worth the increased functionality I get from being able to actually multitask instead of the tacked-on hack that iOS4 uses.
I wouldn't have gotten any use out of my iPad if I hadn't jailbroken it. I was mostly ambivalent about doing so on my iPhone when I originally purchased it, only needing it for the carrier unlock when I moved to Australia (and changing the device's appearance.) I could go either way.
But jailbreaking mine allowed me to pretty much completely replace lugging my laptop around. I have noticed no measurable difference in battery life as a result, and still get phenomenal life out of it. If there's somehow a "huge" difference that I'm missing out on, it's worth the tradeoff to me. If I hadn't jailbroken it I can state that I would have absolutely returned it to the store a few days later after the shiny factor wore off and I realised it was otherwise useless.