I've been using raised beds for a number of years now. I joined mine with lag screws also. 1/4" by 3.5 i believe. I'm going on 7 or 8 years with them and they're still going strong. My thought for using them instead of nails is the nails would work out as the wood swells and shrinks. Lag screws do the job for me.

My raised beds are constructed of 2 X 8 pressure treated lumber. To make the plastic pipe corners, I cut the 1-½ inch PVC pipe into 7 ½ inch pieces. I then drill four 1/8th inch holes clear through the pipe at right angles to each other opposite to where they will be screwed to the board ends using 2" deck screws. In order to insert a 2-inch deck screw, the two holes on one side have to be reamed larger with a 3/8th inch drill bit. These holes are drilled an inch from both ends of the PVC pipe corner

I just ran across something like what you're talking about on flickr: http://www.flickr.com/photos/beeziboy/910010808/

Is that the sort of thing you're talking about?

I'm surprised the PVC gives it enough strength!

Here is a link that might be useful: Hands On Blog: Less Talk, More Action

I just used 3 inch deck screws with nothing special in the corners. That said, I looked at what you did and that will be very strong and should hold up since they're galvinized. I think you could go with less for the future boxes, but if you're good with the effort and cost of doing what you did, then stick with it. They look very well done!

Here is a link that might be useful: My beds...

I love the angles on your beds! Your use of space looks very efficient.

Here is a link that might be useful: Hands On Blog: Less Talk, More Action