The service works much like YouSendIt, though rather than uploading a file, you point to an existing file on your iDisk. MobileMe then assigns a URL to that file, and offers to send an email to a recipient with the link. You can also assign an expiration date and password to the link.
MobileMe's sharing functionality is so far only available only through the iDisk web application, and not through the Finder. Tools like Dropbox and FileChute -- available on the desktop -- allow you to upload files and assign them a public link, but don't feature expiration dates or passwords (yet).
Apple offers a tutorial on how to use the new feature on its website.
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