Yup, FF, your point is valid.

Also, if you cash in after 6 months, you only get 3 months interest.

I worked out my cost of money, less the interest I will earn on these bonds. I also considered the float on my Amex I would get by buying the bond after my account closes but before the end of the month (my card closes on the 20th).

Bottom line, it costs about $0.0142 per mile do do this trick. Not bad, but not great either. Double miles would make this a very good deal, however.