United is running a targeted promotion. Some folks have to fly 3 roundtrips to get 15,000 miles. Some have to fly 4 for 20,000 miles. Others have a chance at 25,000 miles.

Here’s the one that I received.

Book and complete your qualifying travel by June 30, 2008
Enjoy earning more bonus miles the more you fly:

• 5,000 bonus miles if you fly 2 qualifying roundtrips

• 10,000 bonus miles if you fly 3 qualifying roundtrips

• 20,000 bonus miles if you fly 4 qualifying roundtrips

Note that registration is required. All the offers, regardless of which one you’re eligible for, seem to have the promo code MPD048 so the same registration link should work for you regardless.  (If you receive an offer with a different promotion code, please so note in the comments.)

Worth checking your email to see what offer you’re getting, and registering in case you’re eligible for something.