Enterprise Multihoming

My current opinion is that since we can't accept much
downtime in the case of a single provider failure, it's
probably not wise to put all of our eggs in Sprint's basket
even if all circuits are geographically diverse.

Use multiple border routers. Keep your IGP lean and nimble.
Think about BGP/IGP synchronization.

WAN links can fail, but so can ethernet links and entire routers.

We have multiple border routers and are fairly redundant internally. As
it is now, any single piece of equipment could fail (except in one case
that I intend to rectify soon) or any two of our three Internet
connections could fail and no one would notice much except for perhaps
slower connections. I've discovered the wonders of fault-tolerant
transceivers and I'll be redesigning a portion of that part of the
network around them. Once I'm done, quite literally any single device
could fail and no one would notice.