Im trying this:

function BlogEntries($limit=4) {
        $Blog = DataObject::get_one('BlogHolder', "Name = '".$this->blogname."'");
        if ($Blog) {
                $highlight = DataObject::get("BlogEntry",
                        "ParentID = $Blog->ID AND find_in_set('highlight',Tags)", 
                        "Date DESC", "", 1);
                if (!$highlight) {
                        return DataObject::get("BlogEntry", 
                                        "ParentID = $Blog->ID",
                                        "Date DESC", "", $limit);
                } else {
                        return DataObject::get("BlogEntry",
                                        "ParentID = $Blog->ID AND NOT ID = $highlight->ID",
                                        "Date DESC", "", $limit);
                }
        } else {
                return false; 
        }
    }

EDIT: Any way to show code using monospaced font or something similar?

But highlight->ID translates to nothing and the SQL spit it out as Couldn't run query: ....WHERE (ParentID = 1 AND NOT ID = ) AND ...

answering my own question (actually simon_w got it on IRC), worked as

$destaque = DataObject::get_one("BlogEntry","ParentID = $Blog->ID AND find_in_set('destaque',Tags)", "Date DESC");
if ($destaque) {
    return DataObject::get("BlogEntry", 
                                        "ParentID = $Blog->ID AND NOT `SiteTree`.ID = $destaque->ID",
                                        "Date DESC", "", $limit);
} else { ....