I have a 2007 WRX which I bought used with 30k (now ~34k) that began throwing the CEL with code P2444 3 weeks ago. Even thought it was not covered under the extended warranty I bought, I dutifully took it into the local dealership for evaluation. Similar to others' stories, they wanted over $1600 to replace the air pump and both valves. The service advisor claimed there was no way to more specifically diagnose the fault, and all 3 components needed to be replaced.
I called Subaru America, who were *very* interested in this situation. I had a call from the dealership's Service Manager the same day, asking if I would give them another chance to look at the car. In the end, they discovered the pump was fine, and that both valves were filled with water. They replaced both valves for $250 with a notation on the invoice "goodwill warranty."
My guess is that Subaru know they have a problem, and are willing to address the issue if you complain enough.