To be honest, a few Pura Cup matches should be enough for McGrath to find his feet again. He's an experienced player who's been around the Australian team environment and has communicated well with his teammates for a long time, so a few tests throughout 2006 won't really have an impact on how he'll perform in the Ashes. He knows what to do, and has proved his intelligence as a bowler, so if he gets his rhythm right in domestic cricket, he's played enough international cricket to transfer that into the test scene.
But yes, if Clark backs up his recent form, he'll be the back up to McGrath I would think.