Kind of going overboard there redzaku, lol.
I'd be fine with someone running their knife across my throat, if I knew that someone would actually be doing it (eg the game organizer explained the rules would be throat slitting, instead of tapping).
The biggest problem is definitely that when you start wrapping stuff around another person's neck, you have the potential to bump off their goggles. The biggest concern is if they're wearing the mesh half mask. If the mesh gets bumped up, so do the goggles. A paintball mask at least has the full surface that will continue to cover the eyes from head-on impact, even if the goggles get moved upwards.
The second biggest problem is just invading another person's personal space, and the potential for real harm. People might not go as far as stabbing you back with a real knife, but they might instinctively snap their head back, straight into your nose, or something.
As for a helmet cam, it can run you anywhere from $20, to $600+, depending on how good you want.
I got a cheap $20 cam that actually works better than any $20 cam deserves to. It even does half-decent in low-light conditions (it's not good, but you can actually make out what's happening, unlike most cellphone cameras).
On the other hand, you can get GoPros and Contours, with added gadgets and cool crap for several hundred dollars. If you want to make nice videos, you're looking in the few-hundred-bucks range.