At KVM Forum 2012 I gave an overview of GlusterFS for KVM users and developers. If you're looking for an introduction to the GlusterFS distributed storage system, check out the slides:GlusterFS for KVM Users and Developers [PDF]
EDIT: Vijay Bellur provided a link to his presentation on extending GlusterFS. Check it out for deeper information on writing xlators:Extending GlusterFS - The Translator Way [PDF]
Hopefully slides from other talks will become available online at the KVM Forum 2012 website.