While Microsoft is, no doubt, hiding the agenda to further the corporate mission of keeping Windows 8 features under wraps as long as possible, the "shhh" strategy has been a double-edged sword. I've heard from several developers that they were unable to convince their managers that they should attend because of a lack of information as to the sessions.
The Build Web site includes the following description as to the purpose of the show:
There is still no detailed session list, keynote list or speaker list available for the conference.
Microsoft seemingly is holding a number of Build events worldwide simultaneously with the main September 13 to 16 conference in Anaheim, Calif. Among the countries where Microsoft's Developer and Platform Evangelism (DPE) team is expected to hold (at least) one-day events are France, Spain, Sweden, Italy, Poland, Portugal, Ireland, Denmark, Belgium, Greece and the Netherlands, according to Microsoft Community Contributor Francisco Martin Garcia. Dubai also seems to be hosting a Build event.
Microsoft is expected to provide Build attendees with Windows 8 client (and possibly Windows Server 8) around the time of the build conference. It isn't known at this point if these bits will be labeled as Community Technology Preview (CTP) or beta bits. But it definitely looks like Windows 8 bits are going to reviewable by the time of the conference.
Microsoft also still has not revealed a ship target for Windows 8, though it is widely believed to be on track to be released to manufacturing anywhere from April 2012 to late summer/early fall 2012. Company officials also still have not said definitively whether Windows 8 for Intel and Windows 8 for ARM/SoC will ship simultaneously (though that is believed to be the case). Nor have they said for sure that Windows 8 client and server will RTM and ship simultaneously (though this also is believed to be the case).