It may be a little surprising following the Windows Phone 7 and 7.5 release patterns, but Microsoft has yet to release any kind of preview SDK for Windows Phone 8 yet. In past years the developer tools were readily available at least by June, but this go around Microsoft has remained silent. Even during the WP Summit a couple of weeks ago, Windows Phone lead Terry Myerson was only willing to state that the SDK would be released later in the Summer.
Fortunately that's what reputable sources are for, and WPDang (translated) has it on good authority from their own well connected people that the development kit is only a few weeks away. Apparently the SDK release is set for the last week of July, although the site suggests we may see a "preview" or "beta" label attached to the download. If their sources are correct, this would give developers plenty of time before the September-November timeframe, when most are expecting to see Windows Phone 8 devices debut. It would also give us a full-fledged emulator, allowing us to poke deeper into the new OS than what Microsoft has shown publicly yet. We're marking our calendars!
Source: WPDang (Translated)