I haven't seen a definitive answer to this issue so far...
With WP 3.1 The Featured image option no longer seems available in the advanced options for an image within a post, and in earlier versions the only way to make an image appear in the front page image box using Network was to mark an image in this way (except having a default image set with may no be suitable).
I've seen passing mention that just having an image within a post will enabled it to be featured in this way, but with much testing I've not seen that happen.
It seems with 3.1 you get just a 'one shot' chance to set an image as featured, and you cannot retrospectively select an image as featured, which would be the real world case use.
So how do you make an image 'featured' and if that's not required how does it appear in the box?
ps Sorry to have become such a regular!