This is great -- I'm upgrading now for the Disks feature alone. It already works better than MacFusion (no longer maintained?) and ExpanDrive (which is actually $5 more expensive than Transmit!).
This functionality inside Transmit actually depends on MacFuse, it asks you to install it when you select you want to enable the function when you first load it.
MacFusion seems to be maintained to some extent. I just downloaded a version that works on 10.6.3.