I never knew that this program existed. But it seems that it has been around a while.
I got this email today.
For your upcoming flights, it seems that United will offer to let you buy RDM miles (redeemable miles, which don't count to premier status). If you buy those miles, then you can also choose to buy PQM premier miles, which cost more, and count towards your Premier status.
If you travel a lot, then you know that attaining elite status on an airline like United can mean a lot in terms of freebies and benefits. An elite member can board first (thus being able to store a handcarry bag in the upper baggage compartment), get seats with more legroom at no charge (what United calls Premier Economy), and other benefits. The lowest elite tier at United is called Silver, and you attain this status after flying 25,000 miles per year. These are called PQM or EQM miles, and are usually attained only by actually flying. They are different from normal (redeemable) miles, or RDM.
The offer appears about mid-way down the “view current reservation” screen on each of your ticketed trips, and it’s also frequently displayed during the check-in process.
But you can’t purchase PQMs without the redeemable miles, so the overall cost if I went through with this would be $1,872, or 15.6 CPM.
The Award and Premier Accelerator rates fluctuate periodically, and many people receive varying offers for the same amount of miles. A rather active thread discussing this appears on Flyertalk.