Backup Internet Providers?

It's rather hard to answer this question, with out knowing your price range, bandwidth/latency needs, and geographic location.

Somethings to research are,

• Satellite back haul
• 3G/4G (Which it appears you already are)
• WiMax (Clear etc)
• Local Wireless ISPs (more uncommon by sometimes available) (Clearwire is an example)
• FiOS or other Fiber Provider
• Cable Lines
• DSL Line
• Dialup

Some of the above offer very cheap rates for business users because they don't guarantee bandwidth.

Another consideration, if you have systems that need to stay connected, it may make sense to place these systems in co-location that is multi-honed etc.