*Jockeying: The opening heat of the summer movie derby comes Memorial Day weekend, when a motley crowd of contenders will chase after mid-May’s pace cars, Crimson Tide (May 12) and Die Hard With a Vengeance (May 19). First off the mark is Mel Gibson’s nearly three-hour Scottish epic Braveheart, which opens Wednesday, May 24. Friday brings Casper (as in the friendly ghost), the Drew Barrymore-Chris O’Donnell romance Mad Love, and a late entry from TriStar, Keanu Reeves’ cyberthriller Johnny Mnemonic. * Limited Release: New Line Cinema couldn’t resist the blurb Jan Wahl, critic for KRON-TV in San Francisco, offered for Don Juan DeMarco: ”If you can’t get laid after seeing this movie you just can’t get laid.” But the studio did the practical thing with Wahl’s quip, which she never delivered on air, using it only in ads in eight city weeklies. ”The dailies,” says New Line marketing chief Chris Pula, ”probably wouldn’t take (it).” * Bumped: Julia Roberts won’t be one of the queens of summer. Her next two bids for blockbusterdom, Mary Reilly and Game of Love, both previously pegged for August, are being pushed to the fall.