CO is intentionally delaying VB release as he wants to monitor new porting applications(new interfiling applications not in DD). He did that once in past(June2011 VB released on May11 2011).
Jun2011VB:
http://www.travel.state.gov/visa/bulletin/bulle...
Notes From Jun11 VB:
"Since October there has been heavy demand by applicants "upgrading" their status from Employment Third to Employment Second preference. The rapid forward movement of the India Employment Second preference cut-off date in May had the potential to greatly increase such demand. --Therefore, the determination of the June cut-off dates was delayed in order to monitor this demand--".
The above is from a user on trackitt... It might answer ur question....