Beaten to the post! I would still be very careful with US. Be prepared to fight for your account and Burn baby Burn your miles, soon.
Also the offer is not so simple.

Whether you want to buy or gift as many miles as possible, or you're working towards your Preferred status, we have the bonus for you!
From August 16 to September 15, 2010, choose between one of these fantastic bonuses:

Get up to a 100% bonus*
Buy or gift miles between August 16 and September 15, 2010 and you could earn up to a 100% bonus.
Buy or Gift Bonus
1,000 - 9,000 miles 25%
10,000 - 24,000 miles 50%
25,000 - 39,000 miles 75%
40,000 – 50,000 miles 100%

vs

Get up to 5,000 Preferred-qualifying miles!†
For every 10,000 miles you buy between August 16 and September 15, 2010 you'll get an extra 1,000 Preferred-qualifying miles.
Buy Bonus
10,000 - 19,000 miles 1,000 PQMs
20,000 - 29,000 miles 2,000 PQMs
30,000 - 39,000 miles 3,000 PQMs
40,000 - 49,000 miles 4,000 PQMs
50,000 miles 5,000 PQMs

At this rate, 5k PQMs will cost you 575$ - 11.5 cpPQM - a very poor value.
1k PQM will cost you up to 19k x 1.5c = 285$ = 28.5 cpPQM!