While perhaps not the use case that Microsoft envisioned, it's a definite possibility: firms continuing to run their SharePoint 2007 applications and upgrading their desktops to Office 2010.

In reality, we’ve seen a lot of enterprises who held off upgrading to Office 2007 from 2003.  However, many of those same organizations moved ahead with SharePoint 2007 implementations for any number of valid reasons. Whatever the case, the version mis-match is real and potentially challenging as those same customers consider skipping Office 2007 and heading straight for 2010.

Witness, for example, the following error -- when I clicked "NEW" from a document library view:

It just so happens that I have a SharePoint 2007 application, but I'm running Office 2010.  Interestingly, this is the same error you might get if you're running anything other than MS Office or on a MAC.  Other integrations between Office 2010 and SharePoint 2010 -- like the Shared Workspace tool pane and basic document management functions do work properly -- but clearly some functions do not.  If you go searching for other challenges related to "unmatched" Office and SharePoint versions, you're likely to find that this issue isn’t the only one plaguing SharePoint users.

As readers of our SharePoint research know, specific versions of Office and SharePoint are not inherently incompatible.  However, as with any set of independently developed tools (even from the same vendor), there is a bit of "buyer beware" to the implementation.  As such, test careful your scenarios before any mass roll-out; you may be surprised at the results.