I have also had this problem with one of my friends, we both have open NAT's both connecting wirelessy to our router, and we just can't join chat or games toegther, we can both connect to friends and private chat with other friends! It is quite annoying as this is my friend who i play halo 3 team double with the most!

I've heard solutions of just text xbox live connection for both and it fixes it! but yeah, that didn't work for us! any other ideas out thier guys?

ALso could it be an effect from teh bad server issues from christams???