That said, I also think it's silly to sign a contract that states this as a condition of employment. I've walked away from a couple of consulting opportunities in the past because of contracts like this, but gotten a few others because I just politely asked if they would strike out that part of the contract.
I understand wanting to protect your own intellectual property, but I think it's sleazy to claim ownership of things people do on their own time. It's right up there with "no competing with us for 2 years after we fire you" clauses.