Well, the question should really have been, "What is the ideal page width for the iPad?".
The answer is that there isn't one since the iPad, like most portable devices, has two screen widths depending on whether its used in portrait or landscape mode.
If you are designing a layout specifically for the iPad, one solution is to use a folding design where the sidebar folds down to the bottom in portrait and jumps back up to the side in landscape. This will only work for personal computers and iPads.
The popular way of designing a layout to suit a large range of portable devices is to use a liquid, or semi liquid, layout that will automatically adjust to the fixed width browsers used on these devices.
Something like this...
Neither of these methods are possible using a template based editor like iWeb and this is probably one of the main reasons that Apple has given up on the application. Surfing the internet on mobile devices is growing daily and its now become essential to have a mobile version of your website or one that can be viewed on all devices.