Here's how I have it setup for a family of 4, having 3 computers, 2 early model iPods, 3 iTuchs', 2 iPhones, and so far, 1 iPad. ALL on 1 Apple ID account with each sharing purchased items, uniquely on devices and computers:
The key is that you can have up to 5 computers authorized under 1 ID and with HomeShare activated, each computer share and have independent libraries. You can also connect many device types to any 1 computer and 'selectively' sync items to an independent device (where each device has unique items on them). Thus, allowing a single purchase across the board. However, there are some items requiring a new purchase for IPad HD specific.
I have found that though many apps are 'iPhone/iPad' compatible, there may be 'same developer apps' that has been 'redeveloped' for iPad and therefor a new purchase requirement.
The key is that you can have up to 5 computers authorized under 1 ID and with HomeShare activated, each computer share and have independent libraries. You can also connect many device types to any 1 computer and 'selectively' sync items to an independent device (where each device has unique items on them). Thus, allowing a single purchase across the board. However, there are some items requiring a new purchase for IPad HD specific.
I have found that though many apps are 'iPhone/iPad' compatible, there may be 'same developer apps' that has been 'redeveloped' for iPad and therefor a new purchase requirement.