Google Spreadsheets will, at least at first, be able to read and write Microosft Excel files, but will lack Excel's depth of features. It will, however, use tools like Google Talk for online collaboration.
Google further invades its rival's territory Internet search engine introduces own spreadsheet Seattle Post-Intelligencer [seattlepi.nwsource.com]
Google Spreadsheets users will be able to save files on Google's servers and can share them with others, Rochelle said. As many as 10 people will able to edit files simultaneously, and they will be able to communicate with on another as they are making changes.
PS: Looks like longen beat me to it while I was formatting this one. I'll leave it to the mods to sort it out.