I love both the tool in general and the "couldn't sleep so i coded" sentiment. I had the same burst two nights ago when I made http://fstr.net (not nearly as cool as jmockups, but not bad for an evening's work).

Kudos to you! What's more, I may try to redesign my layout in jM. I tried to use a 960gs style "formula" for the buttons and ads... but really just did back of the napkin calculations. This might help :)