Today we noticed that when accessing our OWA with IE8, Office 2007 file types (xlsx, docx, etc) are not being downloaded correctly.
IE8 wants to save them as .zip files.
At first, I thought this might be a IIS server, not having the Office 2007 file types MIME Types setup correctly. But that turned out not to fix it.
The fix? Add your OWA's URL to your IE8's trusted sites list.
UPDATE: Unsurprisingly, the same is true for SharePoint, or at least SharePoint 2003. Same fix.