Sorry to revive a dead thread, but i was curious about this.
I have square for iphone with the free card scanner that uses the headphone jack.
If you had the apk working (before it force closed) did it scan any cards? I thought i saw a download on the market (it's calling the phone function so someone would need to open up this apk) but it looks like it's available for Android:
https://market.android.com/details?i...5zcXVhcmV1cCJd