If you use more than 5GB of data (which does not include purchased music, apps, or books) then you’ll be able to buy more. My .Mac email address is my primary email, I’ve been using it for years, I have all those emails saved on the server, and as of today it only ads up to 2.7 GB. I wonder how fast the remaining 2.3GB will get used once I can store documents and backups as well.

Apple’s MobileMe Transition and iCloud FAQ’s