Neophyte - I also have a DJ-20; what Mark describes is correct (both the adjustment mechanism and the inherent drawback in parallelogram jointers).
I did the adjustment he described on my machine. For me, it wasn't that big of a deal, but I also had an engineering-grade 4' straightedge, dial calipers, feeler gauges, and about 30 years of experience working on cars and rebuilding engines. That certainly doesn't make me hot stuff when it comes to machines, but I simply caution that if you're not someone that has mechanical skill, you may wish to get some help from your local WW club. Just about every one of these has a mechanical expert or 2, and they're usually a friendly bunch that will help you out.
For those looking for a less expensive but still amazingly accurate milled-steel straightedge, you might wish to consider Lee Valley. I have the 36" straightedge, and it more accurate than advertised (they advertise one and a half thousandths over the length). Mine turned out to be about 1/2 a thousandth out over its length when checked against an accurately machined surface plate. Here's the link:http://www.leevalley.com/US/wood/page.aspx?p=56676&cat=1,240,45313&ap=1