On the Facebook HHVM blog today there's a post about the Hack Developer Day they recently held in Menlo Park. The event brought in developers for a day of presentations from the Hack/HHVM engineers.

150+ Members of the PHP and developer community came to Facebook headquarters and joined over 2000 people online for presentations by the engineers of Hack and HHVM. Afterwards we held a five hour hackathon, where the attendees worked with those engineers to write Hack code, either by converting current codebases or writing new code from scratch.

For those that weren't able to attend or are interested in catching up on what was presented, they've posted videos of all of the sessions in a YouTube playlist as well as PDFs of all the slides. If you want the short version of what was presented, there's a quick list in the post or you can read a recap on the Facebook Engineering blog.