For the adventurous, this is how I did it.
Disclaimer, aka CYA: This is only describing what I did. This is not an instruction manual. If you decide to copy what I did, you do so at your own risk. No liability or warrantee by me, implicit or otherwise, is given, etc, etc. etc.
Turns out that I needed two sets of screwdrivers:
A "small" Philip's head screwdriver, and then a really tiny specialized one, (aka a "precision" one, or a "watch" one, or "jeweler's" one) to get the DVD drive cover off. (Approximately a "2.0 mm" head)
First, I unplugged everything, and turned the unit upside down. There are 8 external screws: four under the little "rubber feet" squares, and four under similar squares that are plain plastic. I popped them out easily with my nails.
Once all of these screws were out, and safely stored somewhere, it turns
out that I had to turn it right-side-up again.
I then had to lift the top cover off, from the power-cord side first, and
then slide it off off the joystick ports... being very careful not
to snap the little wires that connect the top to the main unit.
(for the external power push-button and drive tray eject button)
There was a whole lot of dust to get rid of. Once I got rid of ALL of it.. then the tough part began. I had to use the small Philip's screwdriver, to take the four tiny screws off from the top of the the DVD drive area. (NOT the two on the side.. that just holds the tray assembly on)
The drive itself does not come out normally: this just allows you to take off the cover, and get to the lens. Once it was off, I very carefully and thoroughly got rid of all the dust there also. I then closed everything up, and gave it a try.
http://faqs.ign.com/articles/390/390535p1.html says that you can optionally soak a Q-tip in rubbing alchohol, and carefully swab the lens for a little extra cleanliness. I did not go that far.
My unit had then gone from totally unable to read any kind of disk... to reading game disks again. I just saved myself $100 !!
Note on using compressed air blowers: If you plan on using very high-powered compressed air to blow out dust, you may want to tape down the fans, as they may become electric generators, and short something out, if forcibly driven at high speeds by a lot of air.
Random useful link: the built-in "auto-diagnosis" firmware