The easy way to answer this question is to ask Coke if you can use their trademark in your film. You will probably have to submit a script or storyboard or footage. While sometimes the companies will be open to letting you use their product after they read their script, if you don't ask you're opening the door to getting your production company -- you -- asked [sued] to remove the trademark. I would go out of my way to avoid using a trademark without permission -- there are plenty of other ways to add realism to your piece without the legal questions and headaches, including changing your location to use a drink machine that doesn't have a trademark on the front or having your friend drink out of a glass rather than a can or taking the label off the bottle.
I know it doesn't seem like your demo reel will ever be seen by anyone important, but if you do make something interesting and good, you will want to show it off or develop it more. Everytime you show it to someone new, there's a chance that they will think it's soooo cool and show the tape or site to their cousin in Atlanta who works for Coca-Cola. For that matter, if you do turn your clip into a longer piece for festivals, someone with a legal background will have you remove all the trademarks you can, even the ones you don't think you have. I submitted this student assignment to a fesitval: http://www.czarina.tv/rollerskates.html and was told to make a different movie without the trademark. Can you spot the trademark? [I had to remove the word "Grace" from the fake building. There's a "Grace Building" in New York that is owned by a trillion-dollar real estate trust. While the people at the building liked my little movie, there was no-one there who could give me permission to have my fake building say "Grace".]
Before I worked in 3d I worked in film as a graphic artist doing replacements -- basically I made fake logos to go on products that the films did not have permission to use. When I was working on the film Kissing Jessica Stein, we desperately wanted a specific ice cream brand in one of the scenes. However, the film was about a lesbian relationship, the character eating the ice cream was gay, and the company wanted nothing to do with the film because as far as they were concerned, it would hurt their image to have this character eat their ice cream. Since we were negged with the ice cream company, I made a replacement sticker to go over the logo so that it looked like ice cream, but didn't have the real brand's mark. You can use a legally use a parody of the brand -- you can use the Golden Arcs instead of the Golden Arches. If you are making a satirical piece on sweatshops at Nike and want to get your point across without getting sued, you could invent a company named "Mike" or "Strike" and have a swoosh-like trademark for your invented company.
-Mary
