Isolated means not connected by a conductive pathway. Usually this is accomplished by a transformer or by an optical device (vactrol, led/ldr, fiber optic cables, etc.). For example, most wall-warts use a transformer to convert mains AC to lower voltage AC, then a diode rectifier to convert to 9 VDC (or whatever voltage). The ground of the mains power is not attached to the ground of the DC output because the transformer works by induction, not conduction. You end up with two leads that are held 9V apart, but neither is tied to any other voltage. So you can tie one end to ground and the other end becomes +9V. Or you can tie the other end to ground, and the first end becomes -9V. Or you can tie one end to +1,000,000 VDC, and the other end becomes +1,000,009 VDC (or +999,991 VDC if you reverse the leads).
This is why there are no ground loops when each pedal gets its own dedicated adapter. None of them have a conductive path to each other. It's also why you can use the same type of adapter for positive ground pedals as you use with negative ground pedals (as long as you route the leads correctly), but you can't daisy chain positive and negative ground pedals together.
The trouble with DIY-ing this, is that transformers work with AC, not DC. So you can't use the output of a cheap, unregulated, $2, +12 VDC wall-wart that you found on the ground as a source voltage. People generally use mains AC as a source, which is comparatively dangerous and expensive. I suppose you could use an adapter with lower voltage AC output. That would be less dangerous, but just as expensive, as you'd still need separate transformers for each isolated output, or one transformer with multiple secondaries, but with proper voltage conversion for your adapter (so the Weber mentioned by R.G. wouldn't work). You'll also need a separate rectifier for each output. This is all in addition to all the same caps, regulators, and charge pumps you'll use for the AMZ-type supply.
The design of such a multi-supply is slightly more complicated because the load on the separate chains can affect the source supply (an adapter is not a deep ocean of voltage like the mains are), so the supply voltage may change depending on the specs of your source adapter and how much current you draw. Not un-doable, certainly, but more engineering work involved. Of course, if you daisy chain your outputs, you get your ground loop back anyway. So if you have a big pedalboard and you want isolation, you'll need a separate output for each pedal.
So if you have problem with hum that's directly traceable to your power supply, you could consider the extra work or expense of DIY-ing an isolated power supply. Otherwise, it may not be worth the effort. Of course, at that point, the Harley Benton PSU that tjdracz linked to in
the other thread starts to look very attractive.
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