ARRL has an extensive description of amateur radio SDR at: ARRL TIS
In addition, here are recent reports of the ARRL SDR Working Group:
The GNU Radio toolkit for building software radios is available at: GNU Software
Tucson Amateur Packet Radio (TAPR) has a number of SDR projects underway. See: TAPR.org and look for HPSDR.
The Federal Communications Commission (FCC) issued a First Report and Order in ET Docket 00-47, Software Defined Radios (SDRs), amending Part 2 of its Rules to create a new class of equipment for SDRs with streamlined authorization procedures. See Notice of Proposed Rule Making and Order: NPRM. This was followed by ET Docket 03-108, see: ET Doc.
Dr Michael J. Marcus, N4JMM, sought information of the certification of amateur radio SDR equipment and received clarification in document FCC 07-66: http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-07-66A1.pdf. See lead story in: ARRL Letter.
The International Telecommunication Union (ITU) is studying SDR and has published the following documents:
Report ITU-R M.2117 “Software defined radio in the land mobile, amateur and amateur satellite services”
The above Report responds to: Question ITU-R 230-2/8 “Software defined radios”
In addition, work is underway on Cognitive Radio: Question ITU-R 241-1/8 “Cognitive radio systems in the mobile service”
The 2007 World Radiocommunication Conference adopted the following:
Resolution 956 (WRC-07), “Regulatory measures and their relevance to enable the introduction of software-defined radio and cognitive radio systems”
WRC-07 established agenda item 1.19 of WRC-12 (formerly known as WRC-11):
to consider regulatory measures and their relevance, in order to enable the introduction of software-defined radio and cognitive radio systems, based on the results of ITU R studies, in accordance with Resolution 956 (WRC-07)