Here's some harsh advice about spec scripts from Why Television Sucks. I just ran across this blog, which is quite useful. It's by an Emmy Award-winning TV working writer. At least, she's working until someone at the network reads her blog. So I recommend checking this one out sooner rather than later.
I love that site. I wonder how long it will last, though. She doesn't give her name but with the info she provides it's really easy to ID her.