Many SEO experts still recommend not to use multiple H1s, and they may do so with good reason. Maybe it did matter somewhere in the past, but this article is about how it will effect your site today.
HTML5 will be using a new sectioning system, where sections are created using the section element, and headings are made using h1 exclusively. Thats why you should care about how this may effect your site in the SERPs.
Some webmasters are worried that it will trigger signals with search engines like Google, who will then mistake the use of multiple h1s, as an attempt to game the system. Some SEOs think that text inside h1s is considered more important by search engines, and we have seen cases where webmasters would attempt to abuse this to archive higher rankings.
Are multiple h1s invalid?
There are even people who think its invalid to use multiple h1s in html4, but that just doesn't make any sense. The h1 creates a subsection in the sectioning root of a document. In other words, h1 is not the first section on a page. The body is the first section, and this is where it would make perfect sense to markup headings for navigation menus using h1s, as well as the first heading of the main content.
Not that all this matters in practice, it more or less belong in the nitpicking category of semantics. But due to these confusing SEO advices, a lot of pages doesn't make much semantic sense. For instance, if you are using h2s for your navigation headers and h1 for the first main-content heading, then that would indicate that your navigation belongs to the main content, rather then being independent sections of the body.
Think of your site as a big house, where the doors are the navigation elements to each room, and where the first section is the fundament on which the house is build. This would more or less make the walls and doors belong to a subsection of the fundament, (aka root section). Using h2s for your navigation sections, would pretty much make your content become the door to your navigation. I'd make a wild guess, and say this could be confusing for screenreaders.
Multiple h1s good or bad?
What it all comes down to, is whether its bad to use multiple h1s. Brugbart has been using multiple h1s for a long time, mostly for navigation headings, but as of late, also for the main content and its subsections.
We haven't really noticed any negative effects when we adopted the HTML5 sectioning system, and neither did we doing the time where we used h1s for the navigation columns.