May Birthdays 
I happened to see this in the ACM TechNews distribution:
Software Kills Squealing Gig Feedback
New Scientist ( 04/05/08 ) Margottini, Laura
Researchers at the Centre for Digital Music at Queen Mary, University of London, have developed a system that enables bands without sound engineers to give feedback-free performances. Feedback occurs when a specific frequency reaches a critical volume and is re-captured by microphones enough times that the audio system becomes saturated and a squealing sound occurs. A sound check before a performance can reveal which frequencies will lead to feedback, and during performances a sound engineer can adjust volumes to prevent feedback, but the tedious task distracts the engineer from concentrating on the quality of the mix. Automatic software filters can help, but they tend to remove some non-feedback sounds and allow feedback to slip through. The researchers developed software that prevents feedback from occurring, instead of trying to remove it or cancel it out. A sound check is still required to determine which frequencies cause feedback, but during the performance the software automatically lowers frequency volume before it reaches critical volume while maintaining a good sound balance.
Full article at NewScientist
Not sure what you do when you WANT feedback!
