I'm on 705: lga-dfw(dep: 700) 5/26/08 it is still showing f7 a3(expert fllyer) and my upg has not yet cleared(now almost 12 hours before takeoff.) Though my connecting flights dfw-las (and the return via ord) have cleared at the 100 hr marks. From "empirical" evidence, I think aa uses historical data to mete out upg's. A "liesure" route were people rarely purchase f gets upgraded early; if they think they can sell seats they will wait.
[lga ord and lga dfw have more "business types" flying these so they make you wait until the last minute...]
Exactly right. If there is one thing that AA is very good at, it is revenue/yield management. The system is very sophisticated and knows exactly when to release upgrade inventory once the potential for full-fare sales of all available premium seats is statistically gone (based on historical data of that particular flight's performance).
If you really care that much about getting the upgrade early, use miles or an eVIP--especially if you are connecting to international.