I received a slightly different offer - fewer bonus RDMs, but a choice of an RPU or 2 club passes:
To enhance your travel experience this year, we are pleased to offer you a choice of two United Club one-time passes or a complimentary Regional Premier Upgrade. All you have to do is register by May 31, 2014, and you will see them in your account within 7–10 business days.
Also for a limited time, you have an exclusive opportunity to earn double miles on all qualifying roundtrips on United:
• Book your travel by July 31, 2014
• Fly between May 1 and July 31, 2014
It's valid for tickets issued on 4/22 or later. Too bad we already booked our summer tickets to Europe on the cheap-o business class sale a few weeks ago, but I should still be able to get a few miles out of it.