Just noticed this effect. Not sure if this is new since the WP 3.1 update or I just never used an image with a caption at the start of a post before.
I was trying to put up a post last night with an image and caption aligned right at the start of a post. I noticed the post preview showed a blank entry. I posted the entry and it looks fine on the homepage, but when you click the more link or the permalink you go to a blank post! (everything is there...comment form, sidebar, but no content).
If I place even one character or html like , the post displays fine. It took me a long time to figure out what the problem was.
Has anyone else experienced this behavior or heard of it?