Looking at some of these apps, I was thinking that a Stanza-like approach would be perfect. Stanza goes back a page if you tap the left of the screen, shows some controls if you tap center, and goes a page forward if you tap the right of the screen.
Dunno, something like tapping left half shows the answer, tapping the right half goes to the next card, and on the answer screen, tapping left half means you didn't answer correctly, while tapping the right of the screen means you got it right. It would create more space and make the screen look less cluttered.
I also thought that controls on the actual flashcards screens may not be necessary. Shake to go back to card index would do it.
I'd definitely go with a solid app that could do that, create text based cards within the app (no need to go online), have a black background with white text (saves a bit of energy), randomizer, repeat incorrect cards a preconfigured number of times per session, and lock into either portrait or landscape mode.