Our Wireless Data Manager (WDM) handles private-MMP and public-GPRS data sent from our trucks worldwide. We have 150+ WDM boxes distributed in most countries. Their primary use is to serve handheld devices for package delivery. I chose HA so that upstream hosts can connect to a single IP address instead of using clustering. This was after much discussion of other solutions such as load balancing (LB), and disaster recovery (DR).
We have a routing table which we could have shared using DRBD, but we chose instead to use anonymous-ftp for three reasons: 1) we are behind a firewall, 2) less key management, 3) we control the read/writes to this shared file.
Linux-HA (heartbeat) has been embraced by our group since the user interface is simple, solid, extendable and gives 15-second failover performance. We will probably add fencing via a STONITH device in the near future.