What you must be aware of, when designing a Joomla page that should work on iPad and iPhone too? It is enough to have a proper template and the proper tools to display the respective versions when your Joomla site is displayed on these devices?

No, is not. There are couple of other things too you must be aware when you are building your site to be mobile-friendly (especially good looking also on iPhone and iPad).

Flash

pEven if Joomla and Flash works great on a standard stationary computer, both osX and Windows I am generally against using flash on websites. So some can say, that this is another idiosyncrasy of me. Think again! Fact is, that neither iPad or iPhone can handle flash movies. So if you got a slideshow or a game on your Joomla site it wont show up on a iPad. The solution for this is to use Java Script, HTML5 or jQuery. The conclusion is: flash is something you should avoid. But the situation is not hopeless for flash-aficionados: if you can't live without, you can still publish your flash movies in your site, but be sure that to include them not in the content directly, but in module positions which are not visible/shown in your iPhone/iPad version of the site.

Mouse over effect

Don't use mouse over effects on site that should work on iPad or iPhone. Why? Well, since both the pad and the phone uses a touch screen and there is simply no "mouse over" in these cases. This is especially important when you are designing the menu in the Joomla site. Also be sure to avoid whenever is possible to use scripts which are requiring double-clicking. Those also might fail.