First of all, many companies have very specific rules about co-workers dating.  Find out what these are, and use that as your first reference.  Aside from that, there are some things to consider when thinking about dating a colleague.

1.  Are the two of you in a supervisor/subordinate relationship at work?  Even if there are no rules at your workplace forbidding it (unlikely), it's never a good idea to become intimate with someone who has authority over you, or over whom you have authority.  The intimate relationship will undoubtedly influence decisions such as promotions, raises, and performance evaluations, and that's not only unfair to the individuals in the relationship, but to other employees, and the company in general.

2.  Is it going to be distraction from work?  If either of you is going to be inhibited in your ability to perform your normal job duties due to the relationship, it probably isn't a good idea.  If it is something you can keep outside of, and separate from, work, then it may work out fine.

3.  What happens when it's over?  If the two of you are BOTH mature enough to end the intimate relationship and remain colleagues, with no change in the working relationship, this won't be an issue.  If one or both of you is going to have a hard dealing with seeing the other one each and every day, then I would stay away from the situation.

Basically, a good dose of common sense is called for.  If you are going to embark on an intimate relationship with someone at work, some very specific communication is necessary on the front end.  Ground rules need to be laid out at the beginning to ensure that there is no confusion later on.  Remember, your work is your means of providing for yourself (and perhaps your family, too), so don't do anything to jeopardize that, no matter how fun or exciting it seems like it could be.