For the helmets though. I would prefer something along the lines of Vidmaster challenge. And also make it a little bit harder to find out about. Make people research and read code to figure out how to get it. Don't just come out and say it.
The game will launch in Nov of 2012 probably
One of the challenges could be: Complete the final level with all skulls in 4 player co-op on March 19th 2013
March 19th is the 3rd day of the 4th week of the 3rd month (343)