There's a mean streak in Sir Peter Ustinov that I like. It's nice to know that so accomplished a personage isn't above using his acquaintanceships with notable people as fodder for witty, ungenerous anecdotes and impersonations, delivered in Ustinov's distinctive style--ironic, melancholy, and slightly weary, the attitude of a cosmopolite who's fluent in many languages and satisfied in none of them. The movie Spartacus, in which Ustinov soared to the heights of camp outrageousness as a petulant slave master, is the starting point for imitations of Laurence Olivier striving to look regal astride a saddleless horse and Charles Laughton at his most gargoyle-esque; the shooting of Quo Vadis? (Ustinov played Nero) provides an opportunity for tales of cigar-chomping director Mervyn LeRoy; and dinner invitations from former presidents and prime ministers lead to devastating digs at a scriptless, befuddled Ronald Reagan and an ill-mannered Margaret Thatcher. Even Ustinov's charitable work as a UNICEF spokesman is the basis for darkly droll observations about the nature of poverty around the world, as in his account of visiting the wretched hut of a Guatemalan peasant who's watching The Bionic Woman on his TV. And having professed his admiration for the music of Bach (despite the orthodoxy of English schoolteachers who insisted on the surpassing greatness of Beethoven), Ustinov closes this odd-duck evening with a hilarious impression of a Baroque consort--playing on original instruments only, of course--performing an alleged Bach cantata; this marvelous mimic does with one voice what P.D.Q. Bach needs a full band to accomplish. Shubert Theatre, through May 31 (22 W. Monroe, 902-1500). Tuesdays-Thursdays, 7:30 PM; Fridays and Saturdays, 8 PM; Sundays, 3 PM. $14.50-$38.