if you have vSphere Enterprise+ you could consider having two NICs and Network IO Control to manage the traffic. But these really should be 10G NICs then.
Regarding FT - it probably is a good idea to have dedicated NICs here, especially if the apps inside the VM are busy. However, a dedicated FT-related network infrastructure can have a significant impact on cost, so you may want to have a separate charge back mechanism and a respective SLA which truly reflects the power of FT (always on, like a hardware cluster). FT makes sense for applications which are truly mission critical and when the VM only needs 1 vCPU (as of vSphere 5.1). Examples could be home grown applications or any other apps which run on 1 vCPU and where HA is not good enough to provide sufficient uptime.
My 2c.
