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The tech giant didn't reveal what specific tweaks it has made to improve power consumption, but users with the worst problems have noticed the 'location services' icon in the title bar -- a problem associated with Apple's new proximity-based reminders, which have to constantly monitor where you are in order to remind you when you go past the place the reminder is set for.
Proximity reminders that are coupled with a date (for example, "check the oil in my car when I come home every three months") don't exhibit the problem, because location services only have to be activated periodically.
A bug in the automatic timezone function of iOS 5 also caused location services to be constantly activated, as the phone (unnecessarily) constantly monitored that you were still in the same location for timezone purposes.
Thanks to iOS 5's with battery such as IBM ThinkPad X61 battery , IBM ThinkPad X61 battery , IBM ThinkPad T61 battery , IBM ThinkPad X41 battery , IBM ThinkPad X40 battery , IBM ThinkPad X20 battery , IBM ThinkPad R60 Battery , IBM ThinkPad T60 Battery , IBM 40Y6797 Battery , IBM 40Y6799 Battery , IBM FRU 92P1139 Battery , IBM FRU 92P1125 Battery new delta updating system, the update is only around 39 - 55MB depending on your iPhone/iPod/iPad model, and can be updated on the handset itself over a Wi-Fi connection rather than having to be plugged into iTunes.
To the disappointment of many people who are at work with no Wi-Fi access today, the update can't be downloaded over 3G, even despite Australian carriers' multi-gigabyte monthly plans. If you're stuck in that situation, and you're keen to update your device as soon as possible, you can connect it to iTunes and download the full update (around 790MB) and install it to your device over USB.
A bad security hole in iOS has also been plugged -- the one where an iPad 2 with a magnetic 'smart cover' could have its login security code bypassed.
It also squashes several other security holes which have been hanging around since iOS 3 -- visiting a maliciously crafted website could renavigate you to another server, potentially letting another website masquerade as a legitimate one; viewing a document with maliciously crafted fonts could allow unauthorised code to be run on the device; the DNS client in iOS could sometimes send you to the wrong IP address for some websites; and a kernel bug that could have allowed bypassing of code-signing checks has been killed.
DigiCert Malaysia security certificates are also no longer trusted, as that organisation had released certificates with weak keys and couldn't revoke them.
You can read Apple's full description of the security issues it has fixed here.