Here is the response from mobile me support:
My name is Mike from the MobileMe Support team and thank you for your inquiry about iDisk File Sharing. This feature is not currently available with MobileMe.
When MobileMe was announced, iDisk File Sharing was mentioned as a feature; however, the MobileMe webpage noted that all features listed on that page were subject to change. When MobileMe was released on July 10, 2008, iDisk File Sharing was not included. I apologize for any inconvenience.
However, you can certainly use the Public folder of your iDisk to share files. You can access your Public folder through the new MobileMe interface. The new Internet address (URL) for iDisk Public folders is this: