Oops, "inline SQL" is probably a wrong way of putting it. Actually, even in Symfony2 which has an ORM (Doctrine), I sometimes find myself writing SQL queries in code because Doctrine can be so inefficient in some situations. Writing pure SQL makes your code less portable (i.e. tied to a specific database, in this case MySQL). Now I don't think its bad at all to have somewhat complex group/(sub)queries written this way if portability is not a big concern, and if the number of those queries is not as big, it is something you just need to be aware of.

What we're actually talking about here which is really bad is string concatenation without parameterization.

Meaning, it seems the code is not using a database driver. PDO has been the accepted way of doing this. You pass the parameters to PDO separately from the query for sanitation. We can all see why this is bad:

     if (isset($_GET['limit'])) {
         $limit = (int)$_GET['limit'];
         if ($limit < 1) $limit = 1;
         if ($limit > 10000) $limit = 10000;
      }
 $req = 'SELECT * FROM `Money_Bitcoin_Node` WHERE `Status` = \'up\' AND `Last_Checked` > DATE_SUB(NOW(), INTERVAL 6 HOUR) AND `Version` >= 31500 AND (`Last_Down` IS NULL OR `Last_Down` < DATE_SUB(NOW(), INTERVAL 2 WEEK)) AND `First_Seen` < DATE_SUB(NOW(), INTERVAL 2 WEEK) ORDER BY RAND() LIMIT '.$limit;