MATH UPDATE 2012 Nov 26: correlations are moving very far out of whack.
On a window of 4 months + 10 trading days I calculated a correlation slope of -12x meaning VXX traded down much quicker than normal as SPY rose on average. HOWEVER, I also found something on the 2 month + 10 trading day time window: a POSITIVE Correlation of 4.6 - VXX & SPY dropping together. To see everything clearly a scatterplot will be needed log-scale for each axis. Sorry I have no time yet for this, but do try it yourself.
Scatter-plot features are in OpenOffice & LibreOffice, data can be obtained from freestockcharts.com using the disk-platter icon once you specify several chart ticker symbols for 1 output chart & save as CSV for import. (end 2012 nov update text)
edit 2012 10 05 : 4:1 reverse stock split in effect, reading around 34 (not same on all sites) on vxx therefore old figure 23.68/share would now read as 94.72 maximum
Very soon SPY (DIA, dow, s&p500, qqq, nasdaq) will crash