from www.programtrading.com on US equities:

Program Trading Averaged 64.00 Percent of the Volume from Feb 26 to March 2, 2007
Buy programs were 47.37% of the total program trading volume. That is down a lot from the previous week. Sell programs were 52.63%. That is up a lot from the previous week.

Pure index arbitrage was only 7.9% of the total program trading volume, and proves once again, that index arbitrage is only a very small part of program trading and is not relevant for the great majority of traders. Instead, you should be concentrating on the other 92% of program trading that is very important and very predictable.