OK, I checked with the OnePass Service Center this morning. They said you can use the offer as many times as you want through September 15, and get 3K miles each time.
The offer applies to both one way and round trip tickets.
This is going to lead to some incredible mileage earning opportunities such as a trip I have coming up:
SFO-CLE-HPN-CLE HPN 5,322 miles
x125% Platinum Elite bonus: 11,974 miles
+ 8K booked on-line as 2 one-ways: 19,974 miles
+ Delta SkyMiles earned for the cost of the ticket (for AmEx card) - total > 20,000 miles.
Better than the good old days back in the 80's when we got triple miles for every flight for a while.