Brent has a great post on a technique for agile defect reporting.

Basically it comes down to 2 time horizons. The 10 minute and 30 minute horizons.

I was going to leave the following comment, but his comment system seems to be broken at the moment.

Thanks for the great approach for agile QA. Too many times it seems that QA groups get bogged down in the procedures, and can't see what they are trying to achieve (hint - it is not producing defect reports).