I've been playing through Mass Effect, and my character has gone into combat a couple of times now without his helmet on. It seems like it's because I was automatically not wearing it while walking in the Citadel, and when I went into combat it stayed off.
On the inventory screen you can make your character wear his helmet or take it off while in combat in non-hazardous areas. Does this have any effect on damage? For instance, would a headshot be more effective against me if I did not have my helmet on?