The CSS posted above is specifically for the Blogum theme.
I wouldn't really recommend making the header area static... it takes up a ton of space and it seems like it would be annoying on small screens.
For the Twenty Eleven theme, I looked at the demo site as an example http://twentyelevendemo.wordpress.com/ and you could set the header as fixed with this CSS:
You'd probably also want to get rid of the gab at the top of the page or the content shows through when you scroll:
margin: 0em auto;