Links:

Society of Robots: http://www.societyofrobots.com/

Journals on how to build turrets:
http://members.upc.nl/a.kutsenko/index.htm
http://www.colinkarpfinger.com/projects/defcon/

Other turrets people have built:
http://forums.trossenrobotics.com/showthread.php?t=1281
http://www.paintballsentry.com/Default.aspx
http://forums.trossenrobotics.com/showthread.php?t=1109
http://www.burntpopcorn.net/defcon_bot/

Turret discussions:
http://forums.trossenrobotics.com/showthread.php?t=958
http://forums.trossenrobotics.com/showthread.php?t=1340