If it's job-related, unfortunately you can. If the employer can link the job to requirement it's quite doable.
- If a hooter's girl blows-up to 450 lbs, she won't be a hooters girl for long
- If a personal trainer employed by a gym starts chain-smoking he's easily firable
- If you become a strict vegatarian and refuse to train your muscles, hence becoming too weak to lift as part of your job, you'll be toast
At higher levels of employment, it gets *really* nasty. I used to work at General Electric as an executive-band (a.k.a. e-band) employee. You could lose your job for basically any activity that detracted from the company's image.