Mobile Office is a new application for smartphones based on Symbian S60. It is an Office suite for the OpenDocument format used by OpenOffice and KOffice. Momentarily, it's still in development.
It can open files with OpenDocument text (.odt), spreadsheet (.ods) and presentation (.odp).
If you want to try it out, there's a beta version of Mobile Office available for download that'll work until August 31st.