How do touch screens work, exactly? I mean, if the hardware is built so a press equals a mouse button press... well, you'd be halfway there.

In other words, what happens when you run a piece of software that doesn't support touch screens specifically? Do you retain some ability to use it, or is it simply useless?

It's the ease of use I'm refering to,  such as the icon sizes and menus, in your opinion would the game be a chore to navigate with a fat finger ? Not as fat as a iphone hotdog tho

Im clueless if a game has to be built to take advantage of the touch features in a OS,  such as scrolling or pinch to zoom. 

@Mystikmind: I guess that´s not the problem.

The problem would rather be that you handle a touchscreen-device differently; clicking on tiny icons with your fingers just wouldn´t be practical.
Also when dealing with a e.g. iPad you don´t have as big a screen as you normally have for todays desktop computers, so in the end probably the whole GUI would have to be completely rewritten as to make the game playable on a touchscreen-device.

As was already said, todays tablet-hardware isn´t fast enough though.
I mean, even on a decent computer it can take a while to compute a turn/season when you´ve got many units and cities.
Graphics performance problems would pale in comparison to that.
In the end, the whole game would likely have to be redesigned to really be useful on a tablet pc.

Honestly - this might be something for a fan-mod, perhaps in ten years from now, but nothing Stardock should spend any thoughts on.
Unless you didn´t want a more or less exact copy; a quick somehow-Elemental-like crappy-graphics portable game would be less work, though it wouldn´t have much to do with the original anymore.

There are a few fantasy TBS games for the iPad - most are pretty simple and seem dumbed-down for the touch screen (e.g. Highborn).

The best one is definitely Battle for Westnoth, which most of you probably know of for the PC - a great free fantasy TBS with tons of campaigns - very enjoyable. No building, just exploration/capturing/fighting.

It's been made into an iPad game - no longer free, around $5 I think - which is actually very good. In fact, it contains more content than the PC version, including a skirmish mode. The PC just has campaigns (or did last time I played). My 9 year old son loves Battle for Wesnoth on the iPad (and on the PC too), but it is not a dumbed-down version - can be extremely tough.

@Quabar:

Thing I see here is, as said, not the graphics. Todays onboard graphics chips let you play a lot of games, and even portable graphics should not have that much problems with 640x480 VGA-graphics in 3D.
The problem would rather be sheer CPU-power and RAM.
Especially in lategame the computer has to deal with lots and lots of actions every turn, with dozens of cities and hundrets of units.
I honestly doubt that todays atom- or whatever portable CPUs will be able to manage that.

Well, as I said, maybe in ten years or so.