I don't know the exact 07 Mustang order cutoff date, but I'd guess it will be Friday April 13 because the 08 order banks open on Monday April 16. Job 1 date for the 08 is Monday June 25.
At this point I'd get an 08 as it will only take about 1 month longer to get one and it will be a model year newer. HID headlamps are rumored (optional) with, of course, a couple different colors. There will also be a "mood lighting option".