Some users might already know this trick, but it is always good to share it.
I am a new .Mac users since last summer. I was a bit frustrated by the fact that the 10GB storage space associated with my .Mac account are shared equally between Mail and iDisk, considering I am not using .Mac for my emails.
I just discovered an option to modify such balancing, especially for users willing to get more space for their iLife or backup.
First, log on http://www.mac.com, then click on "Account", then "Storage Setting", you will then get access to an interface allowing you to define storage space allocation between Mail and iDisk.
[translation by Linathael]