A friend of mine is worried that changing the font of the H1 tag will cause Google to penalize it (or not assign as much weight to it). He described a situation where small H1 text could be used to fake the important keywords on a page. I assured him that this was silly, but he insists on hard reference. Could anyone link me to something written by Google?
Of course a header in another font has not less weight. What your friend was describing is, that Google will sort out pages, which try to get a better rank for keywords, that are not actually visible to the user.
That means, Google will penalize pages that try to show different content to search engines as to the user. A font size of 1px is only one possibility, you could as well use a black font on a black background. If your page changes the font, but remains visible to the user, that should be no problem.
I missed the point of providing a link to a google article, so the article Hidden text and links of the Google Webmasters page shoud be helpful i think.
@Glycan - You're not going to find something from google that says 'changed fonts for h1 tags are ok', because that doesn't exist. Typeface changes are presentational--search engine optimisation deals with content and code structure. SEO helps users find your page. Good visual presentation helps you keep users ON your page once they're there.
Google spiders aren't looking in your css files to find out whether you've changed your paragraph tag typeface from Arial to Courier-- they're too busy sifting through markup to get at those keywords in the content.