I started getting this error recently on my development box after I started using WSP Builder. I really like this program but I was a little put off by this error. Obvisouly it is recycling the worker process or something along those lines (too lazy to look it up). I am fine with it when it comes back online after a few seconds but I was having instances where it was hanging there...stuck in 503 limbo.
Turns out the problem is the application pool is stuck in a stopped state and the fix is simply enough to restart it.