New Application HA Whitepaper
A quick summary:
The Application HA monitoring API was introduced in vSphere 4.1. HA vendors (such as Symantec) will provide the in-guest application monitoring components, and then these components can integrate directly with VMware HA for intelligent remediation.
Other solutions can either have technical limitations (VM Fault Tolerance) or be very complex and/or expensive (i.e. MSCS) and they don’t integrate with either HA or vCenter. Symantec Application HA lists for $350 per VM and integrates directly with both HA and the vCenter client.