You can send invitations to non-IMail users in the same way that you do for an IMail user. The non-IMail user will still receive an e-mail invitation, but it will also have a text attachment with all of the Invitation information. This attachment enables them to easily import the invitation into their own calendar system or PDA's.
Note: Although the general invitation information will be imported, the user will not be able to confirm or reject invitations.