Yes, several people are using Tarsnap via Cygwin. It's not something I recommend to the general public, but I imagine the readership of Hacker News wouldn't have any difficulty with this.
I'm not sure if it's possible to build binaries using cygwin which will then run without having cygwin installed. If not, this would turn into "port tarsnap to Windows", which is also on my to-do list, but much lower down.
One client I'm dealing with now has health care data needing backup. I wouldn't trust other online backup services with this, but would trust tarsnap. For now, we're using USB hard drives.