I used to dream up witty responses to these comments and entertain myself with them as I commuted to work.
"Well, it's only temporary, and there IS a baby in there. What's your excuse?"
Or I would imagine replying in mock surprise "OH MY GOD! You are SOOO STUPID! I can see your dumb ass coming around a corner even before you're there...."
In response to "Man, are you carrying twins?" (which happened more often than I cared in my second pregnancy) I would want to say "No, but shouldn't you be doing something about that hair loss?"
And, if you really want to shut them up, tell them about your hemmroids (sp??) or constipation, or some general random fact about pregnancy gleaned from the "what to expect" series.
"You seem very interested in my pregnancy, would you like me to tell you all about it? Maybe you'd like to know the last time I had a bowel movement? I tell ya, things sure get uncomfortable after the 3rd or 4th day..."
Coworker: Man, you look huge!
You: "Did you know that pregnancy decreases a woman's immune system?"
Co-worker: *confused silence*
You: "I sure hope you're washing your hands a lot to help reduce the risk of illness in the office."