We just got back from the 7-day Western cruise and WDW today. I'll write up a report later, but I thought I'd offer a quick reply to your question. We did tip the concierge. We were in a Cat 3 room so they did do quite a bit for us: booked spa, verified excursions & Palo reservations, left us reminder voice mails, printed daily itineraries, checked in our kids into the clubs and provided us with a pager in advance etc., etc. etc.
On the last evening we handed Chad (our primary contact) an envelope with a tip which I expect he would share with the two other members of the concierge team.
-VS