I remembered seeing these pants http://www.craftster.org/forum/index.php?topic=278862.0
a while back and while trying to decide how to recon a men's shirt I decided to see if I could make something similar.
These are my version. I put the pants out of the sleeves. Removed the cuffs and attached them to the bottom of the pants.
I finally got some action shots of my son wearing these, so I thought I'd add them. I made the onesie as well out of a kid's t-shirt.