Microsoft have updated their Docx /open XML plugin for Mac office users. The add on requires Office 2004 or Office v.X in order to work and will not work with the upcoming Office 2008 which is due for release in January.
The update extends the expiration of the previous beta though conversions may still strip out a lot of the original formatting and graphics by downgrading to an .rtf file (so much for the open standard file format).
Neo Office for Mac continues to be an alternative for Mac users wishing to open Office 2007 file formats though text edit in OS X 10.5 (Leopard) will open docx files.
I have also used thinkfree office, an online java based office suite to open Docx files quite successfully though the java interface can be a bit sluggish sometimes.