They’ve reviewed a bunch of sites and alert services in this neat article:
“Some Flight-Status Alerts Aren’t So Alert”
The conclusion is that flightstats.com is the top dog.
Another neat feature of the site: Flight Ratings.
With flight ratings you can find out the true historical on-time performance for every single flight on every airline!
you can check your flight status with flightstats.com by texting google (466453) sample “flight aa 2111”
http://googleblog.blogspot.com/2007/03/flying-high-with-google-sms.html