Nisus Express uses RTF. Mellel uses an proprietary file format. If Pages used a proprietary file format, I wouldn't consider it. It sounds like it's XML based instead. So not necessarily standard (RTF, OpenOffice), but readily accessible. That's very good news and it makes Pages a "contender".
[Mitch Cohen] I was also curious if Pages uses XML. While at MacWorld I confirmed this when trying it on the show floor. I created a new simple file and saved it. The file saved is a package, which can easily be opened ('show package contents'), just like Keynote. The file body is saved in a ZIPped XML file within the package. I unzipped the file, opened the XML, and opened it in TextEdit. At least at first glance it appears to be standard XML.
[MacInTouch Reader] The answer is that Pages, as Keynote does use an xml format that is open. I, like Nigel Warren, find it useful to be able to future-proof files as much as is possible, my reason being that I work in the pharmaceutical business and we have a need to keep docs for up to 40 years, so as we move further into a digital age open and easily transferable documents are becoming a killer functionality.
[Walter Ian Kaye] At Macworld Expo, Pages was being demoed by its UI designer. He indicated that it uses the same format as Keynote. (He also liked to clone palettes by a shortcut of option-clicking on one of their tab icons, which I found cool.) I also asked about Word support. He said that Pages can read/write Word files ....