I actually asked the concierge manager at the resort we stayed, and he said it is totally their pleasure to serve the concierge guests, and that concierge services are not tipped positions. He then ran down all of the positions that were tipped positions. He said that while housekeeping is not a tipped position, it is accepted within the industry.
He stated that the particular concierge staff would especially appreciate a thank you card, which I had suggested.
Since my most recent trip was the first concierge stay for us, I didn't want to do anything offensive.
Before I decided to contact the manager, I figured that I was already paying for the extra services.