One day in, and we know little about Apple's publishing plans, other than the announcement that their reader supports the ePub standard.
Given that they already have a couple of "publishing" applications (the word processor Pages, and the Web Editor iWeb) it seems likely that they'll produce some kind of application that will create documents in the ePub standard. My bet would be on the next version of Pages adding ePub support (at the moment it supports Word, PDF and RTF formats.)
Perhaps the iPad's Pages application will support ePub too?
In the mean time, you can check out Adobe's Digital Editions eBook reader application (Mac and Windows) which will display ePub documents.
And for those interested in authoring, there's always Adobe Indesign. For those on a budget, if you have a Mac, you can download a 30-day trial of iStudio Publisher. This application will export to ePub, and I managed to quickly create a basic document that Adobe Digital Editions was happy with. BUT, it appears that it doesn't like to export files with graphics in them (at least that was the case in the ten minutes I spent with it last night!)
If you have a PC, you might try eBooksWriter, or eCub, or Atlantis Word Processor.
NOTE: Before you spend money on any of these applications, just be warned that - at the moment - there's no guarantee that any of them will produce an ePub document that will work with Apple's iBooks application.