This could be a great tool to reach out to a wider market for tarsnap, more home and office users who aren't keen to twiddle the cli. That said, you might have to maintain a functional pre-compiled binary with an easy installer. My guess is that anyone who can compile a program is more than ok with setting up a backup solution that runs from a script, personally i prefer it.
(potentially stupid shit from a person who hasn't seen the interface yet)
You might wanna make sure that keyhandling is made clear and simple to the user (print it, store it safe, exporting, importing, etc), as it is one of the bigger obstacles compared to some other backup services that doesn't offer decent client side encryption.
Great initiative, especially if it can offer a no-brainer function on windows. (rather than going the cygwin route)
p.s. sorry for double mailing you mr. Gulbrandsen, i failed at sending this mail to the correct addr.