According to the internetz (specifically Kotaku), Sony announced Vita's battery life: 3 to 5 hours. Also, that's with default screen brightness, no Bluetooth, no network play (I'm assuming this means no WiFi and 3G), and using headphones.
/FACEPALM
Leave it to Sony to give numbers in only the most vanilla of experiences. No surprises here, really. Real world numbers would probably drop battery life under 1.5 to 2 hours and let's be honest, that's downright insulting for what is supposedly a portable.
I'm a little curious what the **** the point of this thing is. Once again, Sony overstepped the limits of technology. It's not portable anymore because it'll spend most of it's time tethered to a wall. This doesn't make 3DS'sesses battery life acceptable; just less embarrassing.