Lucky you.

...

Or maybe I'm just unlucky. My wife had the exact same model phone and it did the same thing. Now we're both loving the nifty little features of the iOS, like,
"Oh, you have a call incoming, slide to answer..."
"...but I want to decline the call!"
"I'm sorry, your phone was locked when the call was received, so you'll have to either slide to answer, or mute the ringer, and wait until the call is dropped to use your phone again."
"What?! But I need to catch a snapshot of this moment that's happening just now, before it's gone!"
"Oh well. But don't worry. It's all part of the many benefits of the iExperience.

Enjoy!"
...Which is why we jailbroke our phones. (Nifty little apps like EZDecline that put the choice back where it belongs - with the user!) The only downside to that is that if anything goes wrong (to the point where you can't boot the phone) and you have to restore your phone, one of the guaranteed "features" of any iOS software update
that you will be forced to install upon doing an iTunes restore is removing any exploit used to jailbreak the previous iOS software.. so you're stuck until the next jailbreak comes out. There are ways around that (forced iOS upgrade), however, Apple has gotten pretty good at stopping them with their new A5-based devices.