A: The secret link contains 16 bytes of random data from a source suitable for cryptographic use. This makes the search space large enough, that brute-force guessing of links is not practicable. The communication with the Instawallet site takes place over SSL and as such the link is only transmitted in encrypted form.
The link is, however, stored in a browser cookie locally on your computer. Anyone with access to your computer can potentially access this cookie. The cookie is not optional at the moment (unless you instruct your browser to block it), but will be in the future.
Такой сервис не подходит для постоянного использования. А, вот, для того, чтобы, например, купить одну вещь один раз за BTC, не заморачиваясь с установкой проги - это самое то. Получил монеты из обменника, отправил в магазин и забыл. Всё пучком.