The above post by James Hawkins summed up most of my thoughts, so I'll just add a few more:
If you want to date someone within the same dojo, then you need to remember this:
* Keep your lives in the dojo separate from what goes on outside of the dojo. Leave your outside life outside of the dojo, as you are there to train.
* Public displays of affection in the dojo can get very distracting. A quick goodbye peck on the lips to each other or a quick hug is one thing, but a flat out passionate kiss is really not appropriate.
* If something goes bad, and neither you nor you significant other (or ex-significant other) wishes to quit the dojo, then remember to take your bickering outside the dojo. Again, do not distract the others in the dojo. They are also there to train, not to listen to your grievances.
If you are an instructor, and have an eye for one of your students:
* Understand that while you are in the dojo, your primary purpose there is as that person's sensei (or sempai, shihan, etc). At the same time, when you're outside of the dojo, you're an ordinary Joe. As stated above, keep dojo matters in the dojo, and outside world matters outside of the dojo.