Personally, I think that Apple will start using this space for other purposes. For example, I think that Apple will put any content purchased from the iTunes store on your iDisk. This would allow any computer where you have entered your .Mac information, to have access to your content. Also, this would allow an iPhone to content from your whole library, even if it the content is not located on the phone's flash.
Incrementing the space available in each iDisk will cause some problems for a set of the .Mac users. If you enable iDisk Syncing, your computer will keep a disk image of the contents of your .Mac on the local machine. This allows you to keep accessing the content on your iDisk, even when you are not connecting.
The problem arises when your computer recognizes that your iDisk is bigger. It will create a new disk image of the new size with the content from your iDisk. Unfortunately, this will causes a users who have iDisk Syncing enabled to use at least an additional 9GB of space on their computer's hard disk. For me, this change only left 300MB free on my disk.
Apple should use sparse disk images for the local iDisk cache. Then the disk space used would only grow when more space was used on the iDisk.