If you HAVE to be at the the Magic Kingdom by a certain time DO NOT trust the monorail. Don't let a cab drop you off at the Ticket and Transportation Center. The monorail has been iffy at best for the past few years. As long as 5 years ago, I didn't know better and my DSIL was 30 mintues late for her pre-paid Keys to the Kingdom tour. Luckily, they knew the monorail was trouble and let her in a later tour.
There are stories all over the DIS about delays on the monorail, including someone trying to get from the Grand Floridian (where you could walk if not for the canal that they use to store boats) and it took almost 3 hours to get guests there.
If you can't get a bus, take a cab to the Contemporary-it's a short walk.