You can now convert any text file, image or a .doc document into the new Word 2007 .docx format using Zoho Writer. Just import your source file into Zoho and then export that document as a DOCX file.
And like Adobe Buzzword, Zoho Writer also lets you create docx files from scratch inside the browser without requiring Word 2007.
Currently, you cannot import docx files directly into Zoho Writer but that feature may be added soon.
Related: How to convert docx to doc format