Originally posted by Axl_Rose ... I thought feedback was the pickups hearing the amp... ie you can get infinate feedback/sustain even after the string does eventually stop ringing. [/B]

That's a condition called a "microphonic" pickup. It happens when the coil wires are loose enough to move around when the pickup is vibrated by the sound from the speakers. Most pickups have some kind of 'potting' material applied to the coils to prevent this.

If you get 15 seconds' worth of good tone from a note without moving your fretting finger at all, I suspect you're using high gain. The amp is saturating, so that even a small signal is enough to make it sing. It takes quite a while for the signal to fade down to some value too small to drive the amp hard. Try the same thing while playing 'clean' - by comparison, it fades out quite quickly.