In order to connect to it from Ubuntu, simply click on Places>Connect to Server...

The first option you have here is a list of server types, including FTP, Samba, SSH, and WebDAV (HTTP). Select WebDAV from this list, and then enter the information as follows:

 

Server:     idisk.mac.com
Port:       80
Folder:     Your user name (without @mac.com)
User Name:  (leave blank)

Now enter your username (again, without @mac.com) and password when prompted, and you will find your iDisk mounted with an icon on the Ubuntu desktop.