Mailing the story:

ZFS on Linux reaches an important milestone with the official 0.6.1 release. Version 0.6.1 not only brings the usual bug fixes but also introduces a new property called 'snapdev'. "The 'snapdev' property was introduced to control the visibility of zvol snapshot devices and may be set to either 'visible' or 'hidden'. When set to 'hidden', which is the default, zvol snapshot devices will not be created under /dev/. To gain access to these devices the property must be set to 'visible' This behavior is analogous to the existing 'snapdir' property.