With enough scattering, the odds are good that the organization will be able to keep functioning even if one carrier suffers a wide-spread outage. The IT professionals I've talked with aren't using this as a justification for the employee-owned phone option, but most are happy enough to accept it as a pleasant side-effect.
Diversity comes from the same ability that allowed hacked-off AT&T customers to tweet their displeasure: WiFi capability in the cell phone. Right now, only a few phones have the ability to move their communications to WiFi, but it's going to become a matter of growing importance on check lists, with benefit to both the carriers and the customers. The carriers get to enjoy reduced network load from data-hungry customers, while the customers get some level of protection from network outages. It's not a perfect solution, but it's a real step in the right direction, especially since it's still an option for companies that want to standardize on a single phone or carrier for all their mobility needs.
Now, if you're going to move to the WiFi option, there are still issues you need to take into account. One of the most important is daily fees for WiFi access (Starbucks, anyone?) though there are decent options for this. I've written about Boingo in other venues, but I continue to be pleased with the flexibility they offer when I travel. A single monthly fee gives me access to WiFi hotspots at Starbucks, most airports, many restaurants, and lots of other places. It's a good service, and the kind of problem-solver that enterprise IT folks will need to lean on to make their wireless network more reliable.
Occasional outages are almost inevitable -- the important thing is to plan for them, rather than trust to luck to keep your apps safely working.