If you have .Mac already, you are no doubt familiar with the .Mac status page which lists services that are available. (Hopefully, you won’t have to be familiar with the page when MobileMe takes over because there won’t be as much downtime.) Just below the status lights, you will see that Apple warns that the transition to MobileMe will take place on July 9th, from 6pm-12am PT. Nothing will be available except for desktop email clients.
I am sure that Apple is doing all they can to make sure you don’t lose any data in the process, but it might not be a bad idea to back up everything if you have deleted it from your computer.
It looks like MobileMe will be available on July 10th for current .Mac subscribers,
but July 11th is probably when Apple will start selling it officially, and add it back as an optional add-on for new computers. Or maybe it will be on sale tomorrow, since the Apple Store is down! and it is available as a boxed item from Amazon and Apple.
Apple is offering additional storage, above the standard 20GB for a very Apple-esque fee: 20 extra GB for $49/year and 40 extra GB for $99/year.