Because of the 6 calendar month holding period, having a credit card that closes around the 25 of each month would be ideal. You could then buy your bonds on the 26th and not pay the credit card bill for over a month.

One nice feature of Amex charge cards is that the closing date does not vary. Mine always closes on the 20th, and payment is due on the 3rd or 4th..

Based on my float, I can cash in bonds 147 days after actually paying for them. If you can borrow money at 4.93% and it is tax deductible (Fidelity's current margin rate), and the I bond pays 4.08%, and your marginal tax rate is 31%, then each mile costs $0.00943. Not bad. This deal clearly benefits those who itemize and are in a higher tax bracket.

Also, you get a slightly better deal by buying before the end of February, as that month is shorter.

(edited to correct math)

[This message has been edited by bxwatso (edited 11-12-2002).]