Part of the lack of 2012 ZL1 volume was due to some production delays relating to the software code. That downtime meant the factory couldn't build enough 2012 ZL1s to fill all of the orders received, or to reach its 2012 production target, so Chevy has decided to meet its remaining 2012 orders with 2013-model-year cars, which will begin construction in late June or early July. The 2013s filling 2012 orders will only be coupes, since that's all that was available for the 2012 model year.
Chevy estimates it will take 4-6 weeks to fill the remaining 2012 orders, meaning the 2013 Camaro ZL1 Convertible won't enter production until early August 2012. As yet, Chevy hasn't issued any estimated production figures for 2013, but says they will be in addition to the 2012 catch-up order filling. Chevy does say that the majority of 2013 ZL1 production will be in coupe form, and warns dealers to expect their allocation to be coupe-only.
For those looking to get into a 2013 ZL1, there are a few changes on deck, though they're minor and aesthetic only: the black painted carbon fiber hood insert from the 2012 model year will be replaced with a carbon hood insert painted in Ashen Gray, with available color-matched hood and rear deck lid stripes; a new Blue Ray Metallic exterior color will be available; and the optional Chevy MyLink Color Connected radio with GPS Navigation won't be available until regular 2013 ZL1 production starts in August--2012 orders being filled before that won't have the option.
Hat tip to Neal for the info!