We are finally at a place where most the major players in the Hypervisor space support running from a USB/SD Card.
Most servers now come with an internal USB and/or SD card slot should you want to keep the storage medium free from tampering.
Why is this a good thing? it means our development environments are far more dynamic allowing us to quickly test new releases or even completely different hypervisors and configurations.
Now that USB 3.0 has become prevalent and affordable it is a great time to dabble.
I will be writing up my experiences but some forward reading for those that are interested:
VMWare ESXi 5.x
Microsoft Hyper-V 2012 R2
Citrix XenServer 6.x
Though in the case of XenServer i am not 100% sure this is officially supported.