ABC News’ Rick Klein reports: It’s that annual merger of Washington and Hollywood — the White House Correspondents’ Association Dinner, coming Saturday night at the Washington Hilton Hotel. Or, as we know it in Washington, “Nerd Prom.”This year, for the first time, ABC News will be covering the red-carpet arrivals. From Alec Baldwin to Scott Brown, Rahm Emanuel to Jeremy Piven, Betty White to Austan Goolsbee, we’ll find out what Washington is wearing, and what Hollywood thinks of the political prospects of cap-and-trade.I’ll be joined by Ana Marie Cox of GQ Magazine, our regular contributor on ABC’s political Webcast “Top Line,” for all the action from our big dress-up night. And yes, we’ll be Tweeting along the way — follow me HERE and Ana HERE. We’ll bring you the best clips Saturday night and Sunday at ABCNews.com, with more to come Monday on ABC NewsNOW.We previewed some of what to expect on ABC’s “Top Line” with Ana Marie Cox Wednesday. I chatted with the gang at Mediaite’s “Office Hours” yesterday about some of what to look for this weekend. And today on “Top Line,” live-streaming noon ET at ABCNews.com, we’ll check in with Tammy Haddad of Haddad Media, who tracks the weekend’s festivities at WHCInsider.com, and who hosts a famous annual brunch for the boldface names in town.