Have a saved game file from Rainbow Six Vegas or Splinter Cell Double Agent on your hard drive to unlock headgear items unique to each of those games available when you customize and online character.

ICS-PL Helmet: Rainbow Six Vegas saved game file.
J-E FO Helmet: Splinter Cell Double Agent saved game file .
PAS GT Helmet: Rainbow Six Vegas saved game file.
Tactical Shemagh: Splinter Cell Double Agent saved game file.