Hi Tom,
No the system is not designed to share content across sites. The only sharing possible is things like consuming the RSS feed from a feature in one site from the Feed Manager in another site. So for example you could have the blog in one site and have a feed manager in another site consume the RSS feed from the blog in the first site.
If you really want to share content I would question whether you really need multiple sites, another option is to visually partition a single site, by creating different sections with different skins.
Best,
Joe