> only once you've got a margin account and you've gone into debt can they use your shares.

As I read my customer agreement with ETrade, they are allowed to loan out my shares, even if I do not have a current margin balance. (It reads to me that they are pledged as collateral at all times, but I am not a lawyer.) It seems unlikely and inefficient that if I paid or traded my margin balance down to zero that ETrade would demand delivery of any shares I'd previously lent.