Are, most defintitely. Americans say is, even my Irish mate who now lives there has started saying it, for example "Dallas is well tipped for the Super Bowl" etc. To me that always sounds daft as it makes it sound like you're talking about a place not a club/team. To say "Liverpool is" sounds like you're talking about something more than the team, but it's not something thar gets me too wound up, it's just a word after all.