Project 2007 files cannot be opened by earlier versions of office. the converter will be available until the end of the year and most of the new features will not work when it is released... se below from msft's office web!
You have reached this page because the .mpp file that you are trying to open was created in Microsoft Office Project 2007. Converters for opening Microsoft Office Project 2007 files will be available during the last quarter of 2007.
Until then, to open this file using Microsoft Office Project 2003, the person who created the file needs to save it to the Microsoft Project 2000 - 2003 file format.
Keep in mind that if you save a Microsoft Office Project 2007 project to the Microsoft Project 2000–2003 MPP format, you can lose data in areas where Office Project 2007 contains enhanced functionality.
The following types of data or formatting can be lost.