Just got off the phone with Chase Continental Credit Card Department. The rep told me that all Chase Continental OnePass cards will earn double miles on gas, groceries and dining from April 13-June 13. I was surprised by the weird selection of dates. So, I asked again if the promotion and the dates were valid for all OnePass card holders. The rep repeated again that it is valid for everybody and no enrolment is required.
At this point it is YMMV because it puzzles me that Chase has no webpage about this promotion. I would encourage you to verify it and let me know via the comments on this blog post.
However if it is true, this is a very good promotion because there are very few cards that offer bonus miles/points on dining. At least for dining expenditures, the OnePass card would become the best miles/points card during the promotional period.