I was looking at the Zalman ZM80A-HP http://www.exoticpc.com and it says that it requires thermal adhesive for installation. Could I get by with Arctic Silver III non-adhesive? (I want to be able to pull the heatsink off for a future video card).

The ZM80A does not, repeat, does not require thermal adhesive. In fact Thermal adhesive is NOT, repeat, NOT recomended.

BTW, did everyone know that there are 2 versions of the 80A? The earlier one without 9700 bezel cutout and later one with.

It comes with its own tube of thermal paste, although you might want to use your own (better) stuff. There's a lot of contact area between the heat sinks and the heat pipe to cover (in addition to the GPU of course). There's no need for adhesives--the assembly is held together by screws.

How about using some Chomerics Thermattach tape?

I use it for all my removable needs - no mess, sticks well, and transfers heat efficiently.