http://www.theregister.co.uk/2012/01/20/apple_ibooks/
"If you charge a fee for any book or other work you generate using this software ... you may only sell or distribute such Work through Apple (e.g., through the iBookstore) and such distribution will be subject to a separate agreement with Apple."
The restriction is displayed when users run iBook Author, but only at the end of the e-book design process. At which point one might be prompted to go back and read the EULA - which wasn't displayed during the installation of the program:
There is no reason why Apple would not expand it onto other platforms such as a Windows tablet.