One of the touted new features of vSphere is the inclusion of a Fault Tolerance (FT) capability.  As with many solutions, it's a catch-all solution to having highly available VMs.  This blog post just covers some of the limitations around using the FT solution.

 http://vmjunkie.wordpress.com/2009/06/11/why-you-may-want-to-leave-ft-alone-for-now/