My Office 2007 is a genuine version, purchased recently direct from M/Soft and comes with all the licences, bells and whistles that implies ~ I still get that message. It is my belief that it is a new method that M/Soft have introduced via one of their interminable updates to try to make us all buy some other bit of kit from them. Word on the street is that others are also getting this message despite all having fully licensed software. Some think it is a glitch in an upgrade (it seems to have started in the last few weeks) that M/Soft will have to resolve eventually with another upgrade.
On my programme it happens intermittently, some days I never see it and on other it happen every time I open a document. I have complained to M/Soft as have others ~ thus far they have not replied.