Siri stopped working for me several days ago, around the time I upgraded to iOS 5.0.1. Looking online, no one was talking about it nor complaining.
After some digging, I found on the Apple Support Forums, other users reporting issues starting at different points last week. From what I gathered, this issue is international.
What worked for me was to turn of Siri and reset all my settings. This isn't working for all users but it's worth a go if you are having trouble using Siri.