If you have a Norman Rockwell sensibility, you'll appreciate the notion of dining in the dining room. Not every home has a dining room, though, and people who do may be using the space for something other than the occasional dinner party (think home office). The new rule is that you can set up your dining area anywhere you have enough space for folks to move in and out easily; just be sure to provide 42 to 48 inches around the dining chairs.
If you're using a makeshift table, don't despair. Once you have a nice tablecloth in place, no one will care. Just be sure to leave 24 inches from side-to-side for each place setting with about 12 inches front to back. Remember, you'll have a centerpiece in the middle of the table, too. These housekeeping considerations will make your guests more comfortable, and will work if you're setting up multiple tables in the backyard or just putting together a children's table in the family room.
Because your impromptu dining spot doesn't have all the coordinating niceties of a regular dining room, you can help integrate the space by extending the motifs you're using in your design to other areas of the room. If you're using candles, place them strategically throughout the room. Try to repeat the colors of the room's furnishings in your tablescape. If you're using fresh flowers, buy a couple of extra bouquets to place on occasional tables, too.