The Google Docs & Spreadsheets blog officially announced that when you receive an email in your Gmail account, and it contains a Google Docs or Spreadsheet file, the file should have a link to “Open as a Google Document.”
I am not sure if it opens all files or just Google files. I tested emailing myself a Microsoft Word document and Google did not show me the “Open as a Google Document.” A nice feature to add on top of this would be to easily email a Google Doc or Spreadsheet that you are currently working on to others.
Philipp Lenssen first spotted this being tested in early November of last year. This was unofficially reported a few days ago by Google Operating Systems and Googling Google, which claimed that Word documents are supported. It may have not worked with me because I sent a Word document from my Mac.