Unimaginative businesspeople and pols celebrate freshly cemented deals over the dull cliches of the high-end steakhouse. Worldlier movers and shakers seek out Sorellina, Back Bay’s dramatic, South Beach-inflected, white-on-white temple to modern Italian cuisine. Its attractions are many: the crystalline cocktails, the splendid wine list, the passel of imaginative, artily plated pastas and proteins, ethereal desserts, cossetting service and the company of its oh-so-pretty patrons. Creamed spinach is for plodders; Sorellina is for closers.