About a quarter of the way down style.css you have:
#site-title {
float: left;
margin: 0 0 18px 0;
width: 700px;
font-size: 30px;
line-height: 36px;
text-indent: -999em;
}
#site-title a {
color: #000;
font-weight: bold;
text-decoration: none;
}
#site-description {
clear: right;
float: right;
font-style: italic;
margin: 14px 0 18px 0;
width: 220px;
text-indent: -999em;
}
add the bolded lines to your markup
that moves the text off screen, but keeps it in the source code for google, etc....
(EDIT, I keep forgetting to mention this, yes, use a child theme.... if you edit directly on the 2010 theme, you will lose edits on upgrades)