We've taken our children at these ages:
2005 DS was 2.5 months shy of 4, DD was 9.5 months
2006 DS 1.5 months shy of 5, DD was 22.5 months, and I was 5 mths pg with #3
2007 DS 5.5 years, DD was 2.5, and DS was 5 months
2008 DS 6, DD 3, DS 9 months
The children were great on all the trips at the different ages. Our first trip, we went back to room for naps. The second and third trips, we didn't. Our last trip last month we went back 2/5 days for naps, but planned our days so we were back to the hotel early (around 8pm) on non-nap days.
Opinions vary as to whether you should go back for naps or not. Just try and see what works. Only once has one of our children fallen asleep in the stroller at WDW after age 2.
Don't get too caught up in the schedule. What happens at home is totally different than what happens at the World. You can make all these great plans for naps/meals, etc, but in reality, they will probably change. Try to be flexible. I am a total control freak and it was really hard for me to give up being in control of napping/eating/scheduling on our trips. I think I had/have a harder time than the children do.
So I guess what I'm trying to say is be realistic. Since your little one is giving up naps already, I guess I wouldn't worry about naps, but listen to your child. He/she will tell you what he/she needs.
Good luck, and have a super trip to the world!!