Is it possible to purchase an iPhone app now to download later? I was wondering if there is a way (or at least if it is possible) to purchase an app from the app store now, but do not download it now (to save on disk space) and download it later when required. 

Often there are apps on sale (e.g., Lonely Planet city guides) that are not immediately useful. These are great apps, but not immediately useful. So, instead of downloading the app right away and use up disk space, it would be nice to purchase the app and download later when required.
You could purchase the app, then tap on the icon as soon as it appears to pause the download, and leave it paused indefinitely.