It's CPU usage that's the problem, I have a CPU monitor installed and after a while some process is sending the CPU into overdrive, when I do a hard reset it starts off at around 15%, then when the CPU goes crazy it's 70% or higher making the phone warm and the battery drain.
That's exactly what I've noticed. It's fine for a while, but then at some point my phone suddenly becomes very warm and the battery just starts dropping fast. After doing a hard reset (hold power + Home until restart), it is good for a while.
Anyway, do you know what service that is? That may help narrow it down. I'd like to report it as a bug on the dev site.
I had horrible battery life the past 2 days. Today it started to get bad, then at lunch time I completely turned off Location Services. Now I've been sitting at 75% for a good hour or so, so that may have been the culprit. I'll have to see how it goes for the rest of the day.
And as for the "OMG IT'S BECAUSE IT'S A BETA!" people...
we know
It's just nice to narrow down the cause in hope of disabling something, as a workaround, to continue using and testing iOS 7.