iChat sends out connections to an IP address when configuring camera and mic settings
The port is used to "set up Real-Time Transport Protocol (RTP), Real-Time Control Protocol (RTCP)" according to Apple.

According to wiki, "the Real-time Transport Protocol (RTP) defines a standardized packet format for delivering audio and video over IP networks. RTP is used extensively in communication and entertainment systems that involve streaming media, such as telephony, video teleconference applications, television services and web-based push-to-talk features."

Note: iChat became Messages.

Source: http://support.apple.com/kb/TS1629?viewlocale=en_US&amp;locale=en_US