ARRL has an extensive description of amateur radio SDR at:
<a href="http://www.arrl.org/tis/info/sdr.html">ARRL TIS</a>

In addition, here are recent reports of the ARRL SDR Working Group:

<a href="http://www.arrl.org/announce/reports-2007/july/12_software_defined_radio_working_group.pdf">2007 Report</a>

<a href="http://www.arrl.org/announce/reports-2006/july/13b-Software%20Defined%20Radio%20Working%20Group.doc">2006 Report</a>

The GNU Radio toolkit for building software radios is available at:
<a href="http://www.gnu.org/software/gnuradio/doc/exploring-gnuradio.html">GNU Software</a>

Tucson Amateur Packet Radio (TAPR) has a number of SDR projects underway. See: <a href="http://www.tapr.org/">TAPR.org</a> 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: <a href="http://fjallfoss.fcc.gov/prod/ecfs/retrieve.cgi?native_or_pdf=pdf&id_document=6515682751">NPRM</a>.
This was followed by ET Docket 03-108, see: 
<a href="http://fjallfoss.fcc.gov/cgi-bin/websql/prod/ecfs/comsrch_v2.hts">ET Doc</a>.

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: <a href="http://www.arrl.org/arrlletter/07/0504/">ARRL Letter</a>.

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)