You guys do realise that the same amount of battery power is used whether 2MB or 100MB of RAM is in use, don't you? The RAM is either powered up or it is not.
Also, apps that are not in the foreground shouldn't be using any CPU time and therefore no extra battery power.
I used to use a task killer, but stopped because I was tired of obsessively closing down apps after use and I have noticed no overall decrease in battery life or performance. Android is perfectly capable of managing it's own resources and freeing up RAM when it is needed.
In my opinion task killers have a largely placebo effect (the user sees a larger amount of free RAM and believes their phone is running better/faster because of that number)

.