"Since 1937, the American Society for Information Science and Technology (ASIST) has been the society for information professionals leading the search for new and better theories, techniques, and technologies to improve access to information." [About]
I belong to ASIS&T, and it's my belief that they are the organization most committed to supporting information architects as a profession. In particular SIGIA, which is the Information Architecture special interest group, is particularly useful for IAs, and sponsors the essential IA mailing list.
Filed in: IA - Professional Associations