In case anyone needed any more encouragement that the SPG double points promo is indeed a good deal if you are looking to buy miles:

Assume you owe $5000 to the IRS. You pay with SPG Amex before 17 April 2007, but after your April billing cycle has begun. So you now get to keep your cash for an extra, say, 50 days, because the current billing cycle will end and there is a 30 day grace period before you actually must have settled the Amex bill. If you keep the $5000 in a savings account yielding 4.5%, you will earn $30.82 in interest, of which you will keep, say, $20 after taxes for 2007.

The 2.49% service fee means paying the IRS will cost $124.50 extra (which you don't earn double points on, sadly). Subtract the above $20 in extra interest income for a total cost of $104.50. Total SPG points earned: 10124. With the SPG 20% bonus, that is 12148 FF miles, for a total effective cost per mile of $0.0086.

(Of course, this analysis breaks down for overpayments - you lose potential interest on the money before the IRS sends it back to you - but the worst case cost per mile, ignoring the opportunity cost of lost interest, is $0.010375 per mile. If the IRS takes, say, 50 days to refund your money, and you would have kept in an account yielding 4.5%, the total cost per mile works out to $0.01189. Regardless, if you don't need your miles to be EQMs, and your situation matches the assumptions I made, this is far less effort and much cheaper than a mileage run!)