put it in the iDishwasher?
Don't worry about it, my G5 been running fine for the past 4 years with nothing done to it. You might cause damage to your machine.
If you want to get it really clean, then put it in the back of your truck, and then drive through one of the automated touch-less car wash stations.
Once you're done, be sure to spend a good amount of time at the end by the high pressure hot air dryer.
The extra benefit will be the shiny gloss of the metallic case from the wax and clear-coat protectant.
Should be nice and pretty.
Of course, if you want to take the safe route, just do what I've always done. Get a can of compressed air. Then, start at the top and make sweeping motions with the air. Blow through all the fans thoroughly and work the dust you're blowing out of the case by blowing it out.
Also make sure you blow through the holes on the power supply.
Of course, be sure to turn off the computer and unplug it first.
Also, be sure to always hold the can upright so that the propellant doesn't come out (if you hold the can sideways or upside down, then you'll get a wet spray out of it).