I'm taking mine back to the store tomorrow as I've discovered that Siri not working is just a symptom of my phone's WiFi not working properly. Symptoms are:
1. Full WiFi signal shown on the home screen.
2. All other devices in the house (HTC Desire, iPad, etc ...) are all working fine.
3. Can't actually browse, receive emails, etc ... on 4S even though connected to WiFi.
4. Understandably Siri is not working under these conditions.
It's really annoying. When browsing, I enter the search and the blue bar simply stays at the left and doesn't move. Turning WiFi on and back off again makes it work correctly.
To those with issues when they're on 3G, etc ... I would be very sceptical about Siri servers being down as there are a million and one reasons as to why you're not actually receiving data signal. However, also see if your phone plays up on WiFi too.