One kinda solution:
    *  STUN Server
    * TURN Server ( latest draft )
    * Outoing SIP Proxy Server
    * Media Relay Server
    * SIP B2BUA Server For NAT/Firewall Traversal 

URL:
1. http://www.sipcenter.com/sip.nsf/html/WEBB5YN5GE/$FILE/SIPNATtraversal.pdf
2. http://www.intertex.se/upfiles/IntertexSIPWhitePaper.pdf
3. http://www.cisco.com/warp/public/788/voip/voip-nat.html

My Kinda solution (:)):
1. http://www.iptel.org/ser/
2. http://www.ag-projects.com/SER_Media_Proxy.html or http://mediaproxy.ag-projects.com/