Yes, this line from "Ode to a Grecian Urn" is in iambic pentameter. The term iambic refers to the type of pattern or foot used. Specifically, an unstressed syllable followed by a stressed syllable. Penta means five and meter refers to measurements. Thus, pentameter contains five measurements per line. In the above quote, we can separate out the five feet as follows: than ours/ a friend/ to man/ to whom/ thou say'st. In this case, the words "ours" and "say'st" are meant to be one syllable words. Pronunciation and dialect can affect the way we hear words, but in most cases those words would be one syllable.