Worth noting:

Proxmox v4 Beta as of right now uses linux kernel v4. I updated my homelab to it recently & PCI passthrough (per my previous post) stopped working. Asked on the Proxmox forums & apparently an additional option is required due to the newer v4 kernel:

hostpci0: 01:00.0,driver=vfio

Compared to what was required before:

hostpci0: 01:00.0

Apparently vfio is the way to go with v4 kernels. It should be the default option once Proxmox v4 goes GM.

Update (10/10/2015):

This is the default in v4 GM, so the option is no longer needed.