I've created HeadImage Plugin for WordPress. This plugin allows you to set a header image for each post. The plugin will set the header image of any page it displays to the head_image Custom Field value that it finds in the first post of that page.
I admit to not being much of a PHP programmer, so I'm calling this version .6 and I'll see what I can do to extend the functionality of the plugin. It works great if you'd just like a definable header image for each post, and the default style name is set for the Benevolence (WP 1.5) theme but can be easily defined and should work without any problems in WP 1.2.
Thanks Dougal Campbell for his HeadMeta plugin, which I used as a basis when creating this. Without it I'd have no idea where to start.
You can see this plugin in action at my buddy Walter Costinak's Blog -- he's the one who asked if this was possible and I'm glad I was able to figure it out for him.