Actually, that may not technically be true. I believe the power supplied to RAM is constant throughout the whole chip at all times, despite whether it has data or not. So more apps running would not cause a higher power drain due to RAM, but more due to higher CPU usage. And background tasks don't necessarily have to be using the CPU to reside in memory. Using linux as an example, run the "top" command next time you're at a command prompt. You'll see plenty of programs resident in RAM not using any CPU cycles.
So, again, because of the way the Linux kernel works, you really don't need to constantly be using Task Killers to make the phone run more efficiently. From some things I've been reading, killing tasks that may be writing to Internal Memory may be causing some of the lost free space issues because you're killing the task before it had time to to properly allocate/unallocate it's used space. That may leave space marked as used that's actually not in use anymore.
Now, if apps are being run from the sdcard (which they hopefully will be able to at some point) and/or reading/writing data to it constantly, that could possibly cause additional power use.
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