I don't think raised beds are necessary, unless the soil is really poor. I don't have a garden right now... I broke my leg two summers ago (August '09) and now live at my mom's house. There is a woodchuck living in her backyard.
At the last place I lived, in the Valley (L.A. area)... the soil was hard and dry. It's typical for the area. I started growing some native plants (Ceanothus, a couple types of Salvia plants (Hummingbird Sage, Cleveland Sage, Chia Sage, and White Sage), Sagebrush, Lemonadeberry Bush, California wildflowers, etc.; all of those do great with this type of soil and climate), and while I was at it... I thought I would experiment with Sq Ft. gardening and see how this would affect this type of soil.
I didn't even dig at all. I just hoed off the weeds, laid down some papers (just a regular roll of brown, wrapping paper) to prevent the new weeds from sprouting, covered it with a layer of compost. I put in the seeds (in the compost), or put in the transplants (through the paper). I made three plots of these, I think they were 3' deep and 4' wide each. I wanted to see what would happen to the soil after few years of this. They seemed to do fine the first year (or a season, really).
I also dug out a plot of 1' deep and 6' wide for the tomatoes... and since my area also had the ideal climate for the melons (long hot summer), I did the same for those... several varieties of melons that you don't find in supermarkets, like this nice cantaloupe (I forgot what it was called), etc. I put up a wooden frame behind them with some strings for them to climb around. The tomatoes were great, but unfortunately I had to move out before I could try out the melons.
Too bad I couldn't stay at that place to see if the soil would end up transformed with this method... that would've been cool to see.
I live in NJ, now... maybe this year I'll see what I can do in my mom's backyard, with the woodchuck living there. I'll also do some research what the NJ native plants are.
I wish I still lived in CA... they have some really cool plants. Their season is the opposite compared to what people are generally familar with... their growth is in winter (the rainy season), and their dormancy is in the summer (when it's hot and dry). They die when you don't follow this pattern.