I'm sure it will happen again but my guess would be not until awhile. The Mach 1 has only been gone for 7 model years which isn't long. I'm thinking from 2012-2014 they will produce the Boss 302. Then in 2015 we may see a new re-design which will mean no "special vehicles". Then they will probably bring the Bullitt back for 2016-2017 or the California Special (Would be awesome if they would choose the Twister Special or High Country Special though) for 2016-2018. Then after this it will be time to bring the Mach 1 back for a 2-3 model year run IMO.
Don't get me wrong, I would love to see another Mach 1 sooner (this is one of my favorite's), however I just don't see it happening any time soon.