[Lynne]

You would need to center the whole userinfo area:

#sidebar_container.member_summary .blocksubhead {
    text-align: center;
}
#sidebar_container.member_summary .blocksubhead #userinfo {
    width: 100%;
}

And then if you don't want some centered, such as the name, then go:

#sidebar_container.member_summary .blocksubhead .member_username {
    float: left;
}