I backed out of an offer, but never got a response?

You're missing something important here.

You're absolutely right that a firm will put its best interests before yours, so expecting loyalty is foolish. Similarly, your career is your livelihood, and you need to put your best interests first. When a great opportunity comes along, you've gotta do you. Sounds like you understand this part just fine.

What you're missing is the concept of good faith.

illini29:

I see no difference quitting before you even start than quitting later on. Rejection is rejection and if you find something better no one can fault you.

This is painfully wrong. Someone who acts in good faith plans to start a job he accepts and intends to stay long enough to contribute to the team. You shake their hand with the intent of establishing a mutually beneficial relationship. Emphasis on the words plan and intent. Life happens, and if an incredible unexpected opportunity comes up, you take it. But then you acknowledge that, in doing so, you've put someone else in a bad spot, so you show the courtesy of apologizing for it over the phone. That's the professional thing to do. You wasted this manager's time in bad faith when he could have been out looking for another candidate who actually wanted the job. You can only get away with this so many times before your reputation starts to suffer.

Hopefully your new job is far enough removed from the network of your first offer. If so you'll probably be fine. But I'd strongly suggest you reconsider your approach before you burn any bridges in your new line of work. It's a smaller world than you think.