Mikelopez you are exactly right. It's NOT about whether you can get shocked, and everyone probably already knows that PVC, wood, melamine, etc won't conduct electricity. That is obvious. The real point of the matter is a fire hazard.

Serpentmerchant, you are right, there is less of a fire hazard with PVC, but the fire risk of hooking up a conducting material (the aluminum tape) to a live circuit (the flexwatt) is still an issue. And you definitely CAN short out flexwatt!!! You can short out ANY circuit by bypassing the load. The load in this case is the heating element, which heats up as current passes through it. All you would need to have happen to create a fire is for aluminum tape to bypass the load and create a live circuit.

For example, if you are using 4 inch flexwatt, and a 4 inch piece of aluminum tape to secure a probe, there's a chance (over time) of the aluminum piercing through the plastic insulation on both edges of the flexwatt (maybe after bumping it with the edge of the tub hundreds of times. And then you have a live circuit and a SERIOUS fire risk.

The chances of this happening are pretty low, but as mikelopez said, why risk it??? Just look at the horror and destruction of the Pro Exotics fire that was caused by an electrical short.

Conclusion: use duct tape, or some other method of securing the probe to the heating element.

And if anybody doubts that flexwatt can short out, see link below about urine burning through flexwatt and creating a short:
http://burmjunkies.com/showthread.php?7107-Be-careful