Disclosure: I work for Neverfail

Some other things to consider when looking at Zero Downtime would include

1) Impact of storage failure: VMWare HA/FT, Clustering, Symantec ApplicationHA all assume that the storage is shared and will not fail. This needs to be considered.

2) Planned maintenance

3) Site failure, not just a "disaster" but also consider power outages etc

4) Physical systems, are all applications running in VMs or do existing physical servers need to be considered?

Maybe consider that VMware's vCenter Server Heartbeat availability module for vCenter Server is built on Neverfail technology and this gives an indication of how to address downtime.

Regards

Andrew