From the OpenStack List:
The Essex Design Summit is an event for the OpenStack developers community (existing and prospective coders) to gather and discuss the upcoming development cycle. It is highly technical in nature and is not targeted to the general OpenStack public, which will find the OpenStack Conference much more enjoyable.
If you think you belong in the former category, please join us. The Summit is free to attend, but with limited attendance, so we require registration.
More details at: http://wiki.openstack.org/Summit/Essex