I have an Obopay account. They are definitely a legit company and partnered with Amp'd mobile to provide payment service via mobile phone. I signed up under a promo with Amp'd to receive a $25 prepaid MC. Got account confirmation via text message almost instantly, received the card very quickly and have already spent the $25. That's pretty much the limit of my experience with them.

One thing regarding the miles that I would need to confirm: what is the transaction coded as? If you load your Obopay card using a miles CC, is it charged as a purchase, cash advance, balance transfer? This is certainly an important question to answer as miles are typically only gained from purchases...