Sorry mate, you may need to talk to MS about this.
You either have 2 choices. Open the 360 ports here:
http://support.microsoft.com/kb/908874
or give your xbox a static IP and then enable the DMZ in your router with the xbox address (all blocked traffic will be forwarded to the xbox instead of denying entry).
edit
ops i though this was posted this month. sorry a little late i guess.