A thread over at SEO Chat forums named <h1> - how many times ?, discusses how to properly utilize these tags to improve your page structure for search engine ranking purposes.
I'll quote one post from one of my favorite members there, thewatcher:
Lets have a good look at this... HTML is structured so that the Header tag (h1,h2,h3 etc) gives a specified amount of importance to the surrounding text on a page. So in theory you should have as many h1 tags as necessary as long as you have the same amount of important sections on your page.
Usually one h1 tag is enough for the search engine to see that the page is about "widgets", I like to use just one h1 tag on any website but I do use h2, h3 and h4 extensively throughout a website.
One of the most important questions I would be asking is, how much influence on a paragraph does a h1 tag have?
Multiple H1 tags on a website can trigger a penalty if you have other spamming techiniques on your page.