I wrote a test app with the newest Xcode and simulator. It does work with iOS 7, with iPad Air, iPhone 4s, iPhone5, etc. I would imagine that it will support at least 6.0 and maybe even back to 5.0.

EDIT: Tested with 6.x and works.

The keynote says it uses the Objective-C runtime and it compiles to native code -- so one would imagine that it's compatible with older devices and versions. But that is no guarantee.

They said that they were accepting apps built with Swift for iOS 8 and OSX 10.10 so sounds like it won't work with older OS version. They weren't all that specific about it though so I could be wrong.

New language runs on Obj-C runtime. Maybe apps written in Swift can run smoothly on existing OS.

Perhaps the bridging to all the Cocoa libs won't be supported pre iOS8

Not sure. They said apps with Swift code would be accepted once iOS 8 was released, so possibly not.

They mentioned complete compatibility with C so it sounds like it should be ok.

It'll be available for all devices supported by iOS 8, so going back to iPhone 4S.

The runtime of Swift is also the runtime of Objective-C, but the runtime might need some upgrades to fully support the Swift semantics in a safe manner.

EDIT: Correction. It's available also on iOS 7. Just confirmed by Apple. Great :)

Do you have a link for the confirmation?

It was clarified during the "State of the Union" talk immediately following the keynote.

The video should be available on Apple's site (or if not, very shortly).