My boss also often doesn't respond to e-mail in a timely manner. He is responsible for several projects, is out of the office for days at a time (trips, etc.), and gets a lot of e-mail.

Of course it depends on the person and the organization, but I think if you have to get a response from her/him, it's best to see her/him, or leave a message on her/his answering machine.

E-mail is often treated as something that can wait. If you have to get an answer to a question, it's often, in my experience, the least appropriate communications method.

Best of luck with your deadline!

Cheers,
Scott.