This was last year's (2006) Halloween costume for our then 4 month old. Since we had a snowy(!) October, I made it out of fleece. I didn't have a pattern but it wasn't very hard as I used a basic coverall shape (with the exception of the bell bottoms and oversize collar).
The "EP" belt buckle was a flap that closed over the zipper with velcro. To ensure his hands would stay warm when he was outside I created sewn-in mitts that converted to gold lame cuffs.
More detail pics and modeling on my photostream.