The other rider I saw was on a recumbent bike covered with a green perspex casing. It looked very strange and hilarious and was turning heads around Lake Burly Griffin.
There are some pics of something similar here: http://www.retrothing.com/2011/01/pedal-cars-for-adults.html
Apparently they are also called velomobiles.
So it wasn't just the exercise endorphins that had me laughing on my way to work.