This feature will eventually come to your devices.
The microphone is a part of Siri. Just assume that whoever is looking forward to having a lot of voice features on their iPhone 4, iPads, and iPod Touches will be disappointed. iOS 5 is packed with other features, so this is definitely no reason to hate it. Anyone who's followed the boards over the last few days would know Siri is 4S-specific, and that while part of it may be a marketing strategy, I guarantee part of it has to do with processor power.
I do believe other devices will have some kind of voice recognition software, but it probably won't be close to what Siri is.