I purchased the Wixey Digital Planer Thickness Gauge from Hartville Tool:
http://www.hartvilletool.com/product/12148
I installed it on my Delta 22-580 today. Installation was pretty simple and took about an hour or so. For this particular planer it required that I cut a small rectangle from the end panel of the planer. According to the instructions, for most planers, it doesn't require any modification.
Since the device mounts to the end cover panel of the planer, the instructions suggest that the cover panel should be fully immobilized with the use of epoxy. I chose to skip that step figuring I would deal with it later if it turned out to be a problem. What I have found is that if I purposesly move the end panel as much as I can, it only affects the reading by +/- .001", so I'm choosing not to worry about it.
So far, I love this accessory! Calibration is extremely fast and simple. I've run about 10 boards through it now, and I'm finding the readout to be dead-on accurate. Reading the thickness with a rule that measures in 64ths of an inch, I can not find any deviation.
I can already tell this is going to be one of those "how did I live without it" accessories. I can now reliably take off the amount I intend to with each pass, and make the final pass exactly where I intend to. Historically, I have found it somewhat difficult to hit the last pass dead on without overshooting slightly, but that shouldn't be a problem any longer.