This is outrageous! What's next? Bullets not colliding with the players on the opposite team!? I thought this was a building game! [/sarcasm]

But yeah, it's a good idea. We would need melee weapons for this to turn out great, though. I recommend posting it to the Google Suggestion Thingy.

Players on opposite teams should not be able to walk through each other. It makes close-quarters combat really, really awkward.