With the new update for the Pages App that now allows you to save your documents in ePub format which Apple uses for iBooks. It is also the native format for Stanza (which is the Open eBook standard from the International Digital Publishing Forum http://www.idpf.org). ePub is supported in Stanza Desktop, iPhone, iPod Touch, Adobe Digital Editions and some Sony Readers.
Sadly the Kindle does not support ePub format directly, it uses MOBI format, but you can easily convert your ePub files to a format that can be read on your Kindle device. I hear that Calibre does the best job, with a nice interface, easy to use, works with pc’s and Macs, and gets the best results. If you don't already have this free software, created and maintained by Kovid Goyal, download Calibre here. The nice thing about Calibre is that it will also convert many other formats to ePub format as well so which ever direction you want to go with your eBook you have the ability to get it there.
LexCycle has a great article on how to create ePub files from different file formats and lists many different applications that will allow you to create ePub files for you iBooks and Stanza applications.