You can make iChat file transfers, iDisk, and Back to My Mac work better by turning on a setting with Apple AirPort base stations released starting in 2003. Launch AirPort Utility, select your base station, click Manual Setup, choose the Internet view, and click the NAT tab. Check the Enable NAT Port Mapping Protocol (NAT-PMP) box, and click Update. NAT-PMP lets your Mac OS X computer give Apple information to connect back into a network that's otherwise unreachable from the rest of the Internet. This speeds updates and makes connections work better for services run by Apple.
From Glenn Fleishman