I had issues with it on Day 2, right after I left the AT&T Store; I'd needed to swap SIM cards with another number on my account. Siri was working literally as I was pulling into the parking lot, but about two minutes after I left, I discovered it wasn't working any more. I ate lunch to give it a little time, but it still wasn't working. So back to AT&T I went. They were just as stumped as I was and were still asking me the usual "Hmm, well you tried X Y and Z, right?" questions while randomly yelling things into the microphone when all of a sudden, boom, it started working again.
They hypothesized that maybe, since there's a specific "4S" plan in AT&T's computers even though the cost is the same (something to do with HSPA+), perhaps that bit needed to be flipped somewhere along the line to make Siri work (since it won't work with anything but a 4S, after all), and it just takes a while for that bit to flip for Siri when you swap SIMs.
I'm not sure I buy it, since it doesn't explain how Sprint, Verizon or unlocked 4Ses can work with Siri, but I figured I'd post it anyway just in case.