At the moment, there’s no official, Apple-supported way for getting Siri to turn the WiFi (or bluetooth, or the ringer) on or off on your iPhone. This has baffled some, as it seems like a no-brainer.
The reason you can’t do it is not technical, but rather conceptual. The concept of Siri, what it’s meant to be, is a personal assistant. You’re not talking to your iPhone when you invoke Siri, you’re talking to Siri. You are not making changes to things on your phone, you’re making changes to your life.
That may sound somewhat grand, but it’s exactly how Apple thinks. You can see that in the terminology they use when discussing their products. It’s all about the product getting out of the way, about interacting directly with your content. Siri is an extension of that. What remains to be seen is whether Apple can keep to that concept when there are features which average users would expect, missing.