Do magic resistance and flame, frost and shock resistance stack?

If it's a multiplicative stack as others have noted, it should work like this:

Say you have a 40% shield, a 30% chest piece, and a 20% buff...

In a simple additive system, you would have 90% resistance (40% + 30% + 20%). This is a problem because a user could figure out a way to completely mitigate damage and render certain foes permanently harmless, and what fun would that be? Generally systems that use this method implement caps, either hard (user can't score higher than x%) or soft caps (after x%, all bonuses are reduced by y%)

In a multiplicative system the 1st item adds its full value (in this case 40%), but the next item only adds it's bonus onto the un-resisted portion of the damage. This works out as follows: 40% + (30% * 60%) + (20% * 42%) = 66.4% resistance. The result is a much lower effect when stacking items. This kind of system makes large-chunk items more effective than collections of small-chunk items and also guarantees that no one can ever reach 100% resistance, even without the use of maximum-caps.

I decided to test these theories. I used the wolf blood exploit to get my alchemy to 800%+ and then used that on enchantment potions. Now I currently have a ring that protects against 200%+ magic. I still take damage from everyone and on any difficulty past apprentice I seem to take full damage.

I'm literally trying it right now. There is no resistance at all. I'm on the latest 360 update for the game.

I believe that they do stack.I can't definately say they do without testing it (such as giving myself a total 100% fire resistance and fighting a flame antornach) but skill buffs stack so its reasonable to assume that resistance buffs stacks also.