No in general, apple did not set up multitasking in the traditional sense but rather a task switcher. Most applications put in the background will go to sleep after their state has been saved. Only some apps like pandora will continue to run.
And more background processes = more processor load = more battery consumed.
It might not be all that much, but I imagine it's something.
Also depends on the new hardware design, ie. chip, efficiency that may balance in the positive direction. So, we may gain hours hopefully.
Well, I'm talking on the current 3GS.
I'm already assuming they'll be using a similar battery in the new iPhone to the one they've got in the iPad, which should result in at least a 2x increase in battery life, assuming power consumption remains the same.
I doubt they can use the same battery as the iPad. The iPad has two batteries.