Microsoft Access 2010 introduces features that are not supported in Microsoft Office Access 2007. Access 2007 Service Pack 1 (SP1) will not open databases containing these features; Access 2007 Service Pack 2 (SP2) will in some cases offer limited viewing and designing capabilities. Therefore, installing SP2 will help you access more applications created in Access 2010.

For the full picture, take a look at this new page on MSDN.