I too get the muffled unlock sound on occasion. At first I thought it was b/c I had a jailbroken phone, but I'm on a stock 1.1.3 right now and I still get it. For me, there's no rhyme or reason that I can see as to why it happens. But yes, turning off the phone then back on fixes it for me.
From everything else I've heard, if this is the only problem I have, I'll be a happy camper.
Mine does it randomly too and what's even more annoying is the muffled keyboard sounds when you tap letters, etc...
I found that you can get out of it by simply going to your iPod and playing a song and then getting out. It's irrelevant what the volume is set to or how long you play it. Just as long as you hit play for even a second it will get out of it. No need for reboots or resets or even restores. It will come back no matter what so just kill it as easily as possible and anticipate a return soon.
They need to fix it because it happens to me every freakin day! I thought I was the only one! Not that it makes me happy to know that but at least now I can hope that someone will find a way to fix it. I'm currently on 1.1.2. I was on 1.1.1 before. Same problem on both and I can see that another post is indicating that he has it on his 1.1.3.