I've got the new Raised Vegetable Bed for the Organic Garden book out and have the first chapter available to see on the site....check it out....The ebook is only 4.99!

Have a few raised flower beds in my compact garden. I think it definitely give the illustion of more space.

Raised beds require very little maintenance. Each spring or autumn, it's a good idea to top dress with fresh compost and manure, or, if your bed only holds plants for part of the year, go ahead and dig the compost or manure into the top several inches of soil. As with any garden, mulching the top of the soil will help retain moisture and keep weeds down. Moisture retention is important, because raised beds tend to drain faster than conventional beds.

The Wikipedia site has some interesting info about how they are intergrated into a permaculuture system.

Raised bed gardening is a form of gardening in which the soil is formed in 3–4 foot (1.0–1.2 m) wide beds, which can be of any length or shape. The soil is raised above the surrounding soil (6 inches to waist high), sometimes enclosed by a frame generally made of wood, rock, or concrete blocks, and enriched with compost. The vegetable plants are spaced in geometric patterns, much closer together than conventional row gardening. The spacing is such that when the vegetables are fully grown, their leaves just barely touch each other, creating a microclimate in which moisture is conserved and weed growth suppressed. Raised beds produce a variety of benefits: they extend the planting season; they reduce the need to use poor native soil; and they can reduce weeds if designed properly. Since the gardener does not walk on the raised beds, the soil is not compacted and the roots have an easier time growing. The close plant spacing and the use of compost generally result in higher yields with raised beds in comparison to conventional row gardening. Waist high raised beds enable the old and sick to grow vegetables without having to bend over to tend them.

Raised beds lend themselves to the development of complex agriculture systems that utilize many of the principles and methods of Permaculture. They can be used effectively to control erosion and recycle and conserve water and nutrients by building them along contour lines on slopes. This also makes more space available for intensive crop production. They can be created over large areas with the use of several commonly available tractor-drawn implements and efficiently maintained, planted and harvested using hand tools.

This form of gardening is compatible with square foot gardening and companion planting.

Circular waist high raised beds with a path to the center (a slice of the circle cut out) are called keyhole gardens. Often the center has a chimney of sorts built with sticks and then lined with feedbags or grasses that allows water placed at the center to flow out into the soil and reach the plants' roots. The charity 'Send a Cow' is promoting the creation of these in Africa.

Vegetable Garden Bed Construction materials should be chosen carefully. Some concerns exist over the use of timber such as Treated Pine that was traditionally treated using Chromated Copper Arsenateor CCA, a toxic chemical mix for preserving timber that may leach chemicals into the soil which in turn can be drawn up into the plants, a concern for vegetable growers, where part or all of the plant is eaten. If using timber to raise the garden bed, ensure that it is an untreated hardwood to prevent the risk of chemicals leaching into the soil. A common approach is to use timber sleepers joined with steel rods to hold them together. Another approach is to use concrete blocks, although less aesthetically pleasing, they are cheap to source and easy to use.

Public concern over potential hazards of CCA has led the industry to look for safer, less controversial preservatives. A few years ago, one of the producers of CCA came out with a preservative touted as environmentally sound. ACQ®, which stands for alkaline copper quat, is a mix of copper and a quaternary ammonium compound, nicknamed quat. Small amounts of copper and quat do leach, but nothing in ACQ is considered hazardous by the EPA, and no ingredient is a known or suspected carcinogen. The maker, Chemical Specialties, Inc. (CSI), uses only recycled copper in ACQ. The wood is expected to last as long as CCA-treated lumber.