Hang on, sometimes it takes more than a day to get an answer!
Depends on what you mean by "hard." As far as I can tell, there is no way to change the templates themselves, but altering a template and saving it as a page that you'll reuse isn't at all difficult.
EDIT: Looking around a bit, I find this utility for adding custom html to iWeb, and this one for editing templates.
I remember macosxhints.com posted a method for creating cutom templeates in iWeb a few months ago, but it looked quite involved. It might get easier once you get a hang of it.
iWeb was just a way for Apple to promote .mac, if you notice it uses spaces by default in filenames which is not supported by many servers. This leads to frustration on the unexperienced customer who then decides to buy .mac. If you want to use your own templates, download the open source Nvu.