Here's the blog post about it: https://blog.dropbox.com/2013/05/introducing-dbx-dropboxs-fi...
It's also rather sparse on details, but the line about being the first to see Dropbox's new products as the most important thing seems to indicate an intention to use the conference as an announcement platform.
Edit: TechCrunch is also reporting[1] that you'll be able to interact with the Dropbox API team:
> Dropbox tells me the three focuses of the one-day conference will be learning about newly launched features on its platform, giving developers a chance to meet and get help from Dropbox API engineers and designers, and highlighting what third-parties have built on top of Dropbox so far.