Question One: Does the daily point move in the Nikkei225 predict the daily point move in the SP500?
Answer to Question One: Yes, but not much. I ran my regression based on daily prices from Thursday, January 4th, 2007 through Friday, April 11, 2008. It turns out the Nikkei does significantly predict a little over 1% of the total price move in the SP500. The problem is it doesn't predict the other 98%+, so this doesn't really present a trade-able strategy (click here for regression results).
Question Two: Does the Monday point move in the Nikkei225 predict the Monday point move in the SP500? Since a lot of newsworthy things can happen over the weekend, perhaps Monday's Nikkei price move has more predictive power.
Answer to Question Two: Yes, it has more predictive power, but still not enough. Based on my regression results, the Monday price move in the Nikkei predicts about 11% of the total Monday variation in the SP500. This is certainly more significant that my previous test, but it's still not good enough since it doesn't predict the other 89% of the variation (click here for regression results).
Question Three: Does the Monday point move in the Nikkei225 predict the change in price for the SP500 from Friday's close to Monday's open? A lot can happen from the SP500 open on Monday to the SP500 close on Monday, so perhaps a better test is simply Friday's SP500 close to Monday's SP500 open.
Answer to Question Three: Yes, it has some significant predictive power, but still not enough. This regression predicts about 9% of total variation which is better than question one (above) and worse than question two (also above). Unfortunately, this test does not explain the other 91% of the total variation (click here for regression results).
Bottom Line: I cannot think of any trade-able strategies based on these regression results (not yet, anyway). At least this puts my curiosity to rest (for now). Let's hope the other 89% of tomorrow's variation in the SP500 (the 89% not explained by the Nikkei) is great news!