Norm, not to split hairs, but.....no, you should not place your napkin on the table if you plan on coming back, but you do not have to place it on the seat of your chair. Using the arm of the chair (or even the back) is perfectly acceptable.
I don't know if I would feel "offended" if someone left my table, but I sure would wonder...
On my first cruise the 3 year old at the table stuck her finger in my dessert. I'm pretty sure that's on the "don't" list.