Me Too!
I am having the same issues. I pre-ordered my iPhone 5 and got it a few weeks ago. I didn't really realize the problem until last week when I accidentally muted and put a few phone calls on hold.
Now, I've tested during a call, tried using Siri's Raise to Speak feature, and watched as I change the ambiant lighting with Auto-Brightness on. None of these seem to work at all.
I've tried a regular restart, a Reset Network Settings, and a Reset All Settings, none of which seem to have solved the issue. I guess I'll be calling Apple support or stopping by the Apple Store next.
Until I have a chance, I found an app that just records where you touch the screen to leave open during calls. Not a great solution :/