Definitely time to break out a suit, man.
Jeans would probably make you feel under-dressed, if you're concerned about such things.
Black tie (while perhaps overdoing it, imho) would probably be more in-line with the dress code than jeans and a polo shirt.
Personally, I'd wear a suit and tie to the event while escorting my lady in a cocktail dress and jacket (assuming it's close to "chilly").
edit: and, if you decide you'd rather ditch the tie later on, it's always easier to remove one and stick it in your jacket's pocket than it is to put one on in the men's bathroom...
