Oftentimes people will use an image for their header text, and then either use display:none or a negative margin to float the h1 off the page. Matt Cutts, the head of Google’s Webspam team, has officially said that this is a bad idea, as Google might think it’s spam.
As Mr. Cutts explicitly says, avoid hiding your logo’s text with CSS. Just use the alt tag. While many claim that you can still use CSS to hide a h1 tag as long as the h1 is the same as the logo text, I prefer to err on the safe side.