Hello, i am now trying to solve simmillar problem.
I would recomend Cradlepoint CBA750 celullar gateway, it has several options to keep the 3G/4G or Wimax connection alive. Very good compatibility. Multiwan redundancy. PoE so you can place it wherever you want, which is actually my situation because i do not get any reasonable 3G signal inside the building where the rack with controller is situated. But there is another problem with the failover.
Aruba controller is only sensing loss of connection only when physical layer down, so for me when using 651 as a router with ordinary cable modem in front of it, it is practically useless. As I dont need to add any router in front of 651, but it seems that i cannot get the failover WAN to work without some Cisco with their SLA service or Peplink Ballance, which both are sensing the outbound availability and then redirecting it to the backup.
A rebranded Cradlepoint CBA750 can be found as CX111 from Juniper Networks.
Have a look at my posts, maybe it will help you a little.
http://community.arubanetworks.com/t5/ArubaOS-and-Mobility-Controllers/failower-wan-on-651-controller/td-p/53268