When using Office 2007 on Vista with WebDAV shares, documents will be opened read-only. This is due to Microsoft's design.
There is a work-around which will put Office 2007 applications into Windows XP SP2 compatibility mode. This can be accomplished by simply adding a few registry keys.
The keys are application specific (examples for Excel, Word, and PowerPoint are listed below):
“C:\Program Files\Microsoft Office\Office12\WINWORD.EXE”=“WINXPSP2”
“C:\Program Files\Microsoft Office\Office12\EXCEL.EXE”=“WINXPSP2”
“C:\Program Files\Microsoft Office\Office12\POWERPNT.EXE”=“WINXPSP2”
You can find more information on this issue here: Microsoft SharePoint Team Blog: Office 2007 on Windows Vista prompts for user credentials....