Theory / 2 Cents: don't just work on a GUI for spaced repetition cards, invent an open file format for spaced repetition (perhaps an XML plus an schema, or a JSON based file format, a subset of HTML or markdown, etc...). The idea is that you create a file format spec, perhaps even some open source libraries to operate on the file format. And then you sell a really nice GUI for editing that (perhaps a web page, perhap…
How about a directory (or zip file) of .txt files for the format of the cards.
I'm working on my own flash cards app (see my other comment) and decided to do something similar. I actually want people to create tools to create new decks. I think the ideal is something like what Anki has in Ankiweb, a place to share decks with others.