In a post to the Windows Team Blog shortly after the announcements today, Microsoft confirmed the Windows Phone 8 SDK will be available in the near future, but only to a select group of developers.
According to the post, the SDK will not be available until Microsoft "publicly unveils Windows Phone 8" (which we thought they were doing today) at some point later in the year. Only current Windows Phone developers with published applications in the Windows Phone 7 marketplace will be able to apply for the Windows Phone 8 SDK this month.
The kit will be in a near-final form, probably a release candidate when given to developers who apply, so it's clear Windows Phone 8 isn't quite gold just yet. There'll be more details next Wednesday, so we'll keep you in the loop then.