Did you ever try to deploy an Access 2007 application with A2007 Wizard if your application uses ActiveX from Visual Basic? You cannot succeed since the wizard ignores ActiveX & similar stuff. Come back, land on the Earth and read my opinions which have been collected to a Bugs & Wishes database: http://www.alis.cz/relax/download/access/Access2007_bugs.rar
IMHO, first of all, Access developer team should fix all known bugs. Then they can show us porting 1 or 2 table "database" to SharePoint. But do you think that "2-tables-database" is a real world? Access in it's version of 2007 became a toy for so called "power-users" who don't know what database is. I'm affraid that those users will NEVER go to SharePoint.
And finally, why SharePoint - a new unknown & buggy technology from Microsoft? Why they can't support Web Services which rule the Internet and will rule it in the future? No, real admins won't go to SharePoint, trust me. They will not port their fully functional applications to an unknown (and probably very expensive) SharePoint.
Also see SharePoint Nightmare: Installing the SPS 2010 Public Beta on Windows 7": http://oakleafblog.blogspot.com/2009/11/sharepoint-nightmare-installing-sps.html which was mentioned at http://blogs.msdn.com/access/archive/2009/11/24/install-access-services-on-vista-or-windows-7-x64-os.aspx#9929508.
Why should we use several back-ends to make a real database solution? BTW, it's impossible to create relations between tables within different databases. Will such solution work? Will it work fast?
Ribbon? One of the worst thing from Microsoft. ("Windows Registry" is No.1). You don't need to use menus & submenus if you create toolbars (see A2003 and older). Ribbon consumes big part of the screen. With ribbon you must re-design your old forms to fit to smaller area. Ribbon? NO! Please...
QAT? Too many bugs, QAT doesn't render correctly, very difficult to use it with VBA (same for Ribbon).
Navigation pane? I'd rather call it "Navigation Pain". I'd prefer former "Database Window".
Compatibility with older versions of Access? Again, see http://www.alis.cz/relax/download/access/Access2007_bugs.rar