Well the VM's are running Ubuntu but the idea of a "virtual appliance" is slightly different than a traditional virtual machine in that you needn't think about the OS. Instead you focus on using the application and for all you know it's BSD or Linux or whatever under the hood but you're managing it via a web interface so you don't care.
to your 2nd question- JumpBoxes support the major virtualization technologies right now: VMware, Parallels, Xen, Virtual Iron, MS Virtual PC and Virtual Server. They'll also run on KVM, Qemu and Virtual Box though we don't officially support those.
I'm glad you dig what we're doing. If you try one out and really like it I would welcome any suggestions you have on how to better spread the word. We have this weird challenge where the people who use it rave about how great it is. We're not at a loss for a killer product- we're challenged in how we spread awareness of its existence.
thanks again
sean