Most computer jobs seem to have a tendency to encourage an unhealthy lifestyle. To start out with, it is not uncommon to sit all day. Add in jobs that reward a level of focus where you forget everything to concentrate until you surface starving only to grab the closest quickest food (often vending machine candy and soda), a geek culture that almost fetishizes coffee and cola, long hours or 24 hour on-call and you have a prescription for potential disaster.
When this unhealthy lifestyle is discussed, it is usually in terms of weight, not healthy habits. For instance, a colleague told me a joke once about how you can tell us server administrators from the desktop administrators because a server administrator needs a layer of fat to keep them warm in the over air-conditioned server room and a desktop administrator stays thin because they are running around from desk to desk and need to be small enough to crawl under desks to fix computers or pull cables. However, when someone talks about doing something about weight, they usually talk about healthy habits – like going to the gym or cutting down on soft drinks. Despite this common use of the term weight as a stand in for healthy habits, most people in the computing industry seem to be among the more accepting of diverse body shapes and sizes.
Computing professional often try to engineer their way to healthier lifestyles or use science and technology to support their health goals. I had a colleague who started taking short jogging breaks at regular intervals because he'd figured out that this was the most efficient way to meet his fitness goals. The most striking example of this is The Hacker's Diet by John Walker the founder of Autodesk, describing how he lost and kept off approximately 70 pounds by treating his weight as an engineering problem. I'm currently experimenting with using Sensa, which is built on scientific research into how taste and smell affect hunger and satiety, to eat better and lose weight. Playing Wii games that involve physical activity as a form of exercise also garners a lot of interest in the tech community, although I don't know a lot of people who are actually doing it.
In general, as a tech professional, you will have support from your colleagues for anything you are doing towards living a healthier lifestyle, but you will not be denigrated for being overweight, out of shape or sedentary. On the other hand, you are likely to be in an environment that encourages unhealthy habits.
Walker, John. The Hacker's Diet. 4th Ed. (self-published: 2005.) http://www.fourmilab.ch/hackdiet/