Also. I just re-read the T&C to find out what is the "minimum funds" required and this is what I found:
Offer valid for new Individual or Joint accounts opened and funded by U.S. residents with $2,500 or more by 06/30/2011
and
Account must remain open with minimum funding required for participating in the offer for 9 months...To qualify for the second half, TD Ameritrade account must remain open with minimum funding required for participating in the offer for 6 months from the first posting date.
It states that the minimum funding is $2,500 and that it must remain open for 6 months from the date you first posted the initial deposit.
So it would seem that once you make the initial deposit and are credited the 1st half of the miles you can withdraw down to the minimum $2.5k and receive the 2nd half of your mileage in ~6 months.
I have just withdrawn down to $2.5k and according to the T&C in 6 Months from my initial deposit I will have the 2nd half.