I hit the top level and just received my email with the following:

Congratulations on your Mileathon success. You’ve earned enough credits to select a Mileathon reward. Check your final progress report and choose a reward by April 6, 2007. After that date the reward will default to the bonus-mile option.

Thank you for participating in Mileathon.

Pretty sure I'll be selecting miles but still thinking about it.....