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've been using it on Cygwin since the beginning. Frankly, its fairly straight forward. The 'configure, make, make install' works out of the box and afterwards you do the same thing you'd do a *nix box. I've had no trouble at all.
Pre-compiled clients are on my to-do list. Obviously I want to do this is a systematic manner so that my release process is more repeatable than "find boxes running the following operating systems: ... and borrow them for a few minutes to build a binary".
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.
With MinGW you can cross-compile from Linux (and perhaps also from FreeBSD). Imho Cygwin is not suitable for applications like tarsnap that should be very reliable and fast.
When you finally do have a pre-compiled windows app, sans cygwin, I'll have more clients to refer to tarsnap.
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.
