Dave's system will give you excellent control over how much to water each pot, but there will be quite a bit of 1/4" tubing behind the rack.
Where the containers are longish and have only small gaps between them, you might have fewer 1/4" tubes visible if you ran some 1/4" dripline with 6" emitter spacing the whole length of one 18 foot row. And you would only need one hole in the 1/2" line and one barb per row (instead of one per pot or 1/4" Tees).
http://www.dripworks.com/categ...
You might be able to scrunch the dripline into a serpentine shape with U-shaped staples going into pots, and prevent any emitters from winding up between pots. Or, run the dripline in the least conspicuous straight line you can, and plug any emitters that fall between pots using something like waterproof silicone glue. It might not stick to the polyethylene very well, but any that oozes into the emitter itself will surely plug it closed.
If you have any very, very thirsty pots, you could use some "Spot Spitter" spray stakes for a higher flow rate. But they are more for watering entire tree root-balls.
http://primerusproducts.com/
http://www.dripirrigation.com/...
http://www.growers-inc.com/wat...
http://www.dripworks.com/produ...
My first thought was to leave 2or 3 90-degree jet sprayers in front of the wall on tall stakes, and spray the whole wall somewhat indiscriminately (leaving water on the leaves and not reaching the soil very uniformly). Dave's way with drippers will be better for the plants and less visually intrusive and waste less water.
