Someone tried to float a scope at work. I then had to help them remove the scope from the grounded box they had it sitting on with a crowbar and saw after the rubber feet failed and the scope spot-welded itself in place.
Remember that not only will metal parts of the scope be hot, but the insulation on the rest of the scope (and mouse and keyboard, etc.) aren't designed for high voltage isolation. Also, don't try connecting the ground clips of different probes to different places or the probe cables will light up.
If you must float a scope, use one of the battery powered ones that are designed for floating; they are aimed more towards electricians than electronic design, but they would be safe. Otherwise, float the DUT and use diff probes.
/mike