Linear hall sensor to detect large fields with digital output?
If you can drop the digital interface requirement, allegro makes a line of linear hall effect sensors that are very inexpensive and readily available. they do have a number of digital options as well, but I haven't used them:

http://www.allegromicro.com/en/Products/Magnetic-Linear-And-Angular-Position-Sensor-ICs/Linear-Position-Sensor-ICs/A1324-5-6.aspx