I am not sure I really agree with this, the double insulation mark indicates that there is two levels of insulator (case materials, fiberglass / mylar sheeting, cable insulation etc) between the user and anything live. A double insulated appliance (such as most consumer electronics / appliances) should
not need or indeed have an earth connection. For more info on that, see here :
http://www.double-insulated.com/
Now, if you want an electrically isolated output the only true way of doing this is galvanomically - either an old style 50Hz transformer / rectifier (heavy power brick kind of thing) or a good quality switching supply with a guaranteed isolation level. You can then pass through the true earth to your chassis and, since it is isolated, peg the neutral output to earth as well - though I would argue this actually reduces the safety somewhat, I would always prefer to leave the isolated supply floating, that way you can contact either conductor and be safe.
Hopefully I have not spread any inaccuracies here, I recently did a large commercial project involving many 3kva isolating transformers so had to read up a bit on all this, but that was a few months ago now...
- Alex