Well... I believe this unusual-extremely-fast-drain-out-battery issue is caused mainly by Push Notification feature (to me at least).
Why I said so ? Here's my situation.
Yesterday I played games on my iPhone and it was about near 20% left, so I stopped playing and planned to charge my phone today.
In the morning, my iPhone battery is near to "Red" level.... and I charged it only about 2pm.
While I was charging my iPhone, I did turn on iPhone's WiFi to update some of my Apps. I turned off WiFi after the updates completed.
So, I left my iPhone charging with my laptop and do my stuffs. After 1+ hours, I checked back my iPhone charging status, and surprisingly
it is in critical RED battery level instead of Green.
Without any doubts, I checked the "Notifications" settings... and I found out that "Tap Tap Revenge"'s Notifications (both Alerts+Badges) are ON !
I remembered that I OFF them before this right after I updated "Tap Tap Revenge". Anyhow... for my case, I believe the Push Notifications have
certain caused the battery-drain-out-extremely-fast than you could think of.... Furthermore, it is extremely
HOT even you can feel it with the casing on it.
My suggestion to those who encountered similar situation, better to :
1) Check your Notifications Settings
2) Immediately UNPLUG if you are charging (cos of overheating)
3) Make sure Notifications are OFF (if unused) before chargining
4) Constantly check your charging status while charging to prevent it is draining out instead of charging
Hope the coming FW 3.1 will resolve this issue too.