One of the interesting things I have done recently is used a F5 Big-IP load balancer in conjunction with an OCS mediation server to load balance across multiple PSTN gateways. While this is a Microsoft unsupported configuration for OCS, it does indeed work and works very well. This configuration of course works only for outbound calls from the mediation to a gateway of some sort. In our configuration, we used it to alternate route outbound 911 across three gateways in a specific order. Something one mediation server is incapable of doing.
Using the F5, we created priority groups and placed one gateway in each one. At the mediation server, we pointed the PSTN gateway next hop to the F5 VIP for the priority groups. The F5 is capable of a SIP heartbeat check so it knows when one of the gateway goes down which then forces it to use the next priority group/gateway in the list. I am not sure what other applications this would be suitable for but with the current limitations surrounding the mediation servers ability to only contact one PSTN gateway I thought it was note worthy.