I imagine that the range of styles and formality will run the gamut from fairly conservative to very outre. Really, you should consult your friend/date. If she or he has no opinion whatsoever, I think that a slim, fairly casual, Helmut Lang-style black suit is fairly standard. Von Dutch and Carpe Diem are both streetwear brands, so you might want to downplay the suit with an interesting top. I've seen hoodies worn under suits to good effect lately, but that may be a little too last season. Maybe you could use the pinstriped sportsjacket and combat pants combination that some designers have been advocating recently to cover all your bases (combats are pretty big this season, so you'll be safe on that front.) However, since you are not one of the grandees, I would stay away from an all casual ensemble unless your date/friend says otherwise.