hey brownsfan, I think the ultimate for this stuff would be able to do it on a basket of actual stocks. That might be a little harder on S&P but for using it against on YM that really shouldn't be much of an issue. I wouldn't doubt alot of what would mess things up with SPY is how much volume is arbitrage against the underlieing.
today he has a cool post too that would make an interesting indicator.
http://traderfeed.blogspot.com/2008/...market_17.html
Plot the volume and range on a 15 minute chart, then take some kind of moving window to get an average 15 minute volume to guage if we are above or below "normal" volume.