Google has added Docs support for Microsoft Word (.doc and .docx). When you receive an attachment with a document in one of these formats, you now have the option to open them in Google Docs. I could have sworn this feature already existed, but apparently that functionality only worked with other document types.
The new feature works in the vanilla Gmail, as well as in Google Apps (all editions). To use this feature, simply click "View" at the bottom of a message that contains one of these documents, and you will be whizzed off to the Google Docs viewer for your attachment.
It's great that Google is building out support for all these document types -- it really does make "Going Google" easier. If Microsoft wants to combat this, they are going to have to start offering similar features -- like the ability to open Google Docs directly into Word.