[Update 5/14/12] Now this offer has been slightly enhanced to 55K after the 1st purchase. You still need to go into your United account to see the offer. It is branded as "65K miles" since you will earn another 10K miles after spending $25K in a year. However this feature is built into the card - meaning you will have it every year you keep the card open, not just the 1st year.
[Original Post] Since Chase introduced the new United Mileage Plus (MP) Explorer Visa last summer, the best signup offer open to public has been the "40K offer" which gives you 25K miles after 1st purchase, 5K after adding an authorized user, and 10K after $25K spent annually. Since the last one is a standard feature of the card, I think the ad is kind of misleading and consider it as a 30K signup offer.
Now Chase have a new "60K offer" which gives you 50K miles after 1st purchase, and again 10K miles after $25K spent annually. This offer is not strictly open to public, however, as long as you are a MP member you are eligible (the registration is free and takes like one minute). You don't really need a link - just go to united.com, login to your MP account, then go back to the home page and you will find the 60K offer somewhere on the page. Note if you are not logged in, you only see the standard 40K offer. If you really want a link here it is but you still need to login otherwise it will only show 40K.
So if you have never had United credit cards, it is a good time to have one. The first annual fee is waived, saving you $95.