The article Do Sensors “Outresolve” Lenses? at luminous-landscape.com is an excellent (albeit highly technical) piece on the optical requirements and limits for today’s high-resolution digital SLRs. It substantiates in detail what I’ve been explaining for years in this blog and in my articles and reviews. See my Diffraction articles for just one example, as well as various blog entries over the years [latest] and reviews like Zeiss ZF Lenses.
Bottom line in all of this: you can use your own eyes to figure out how your particular camera/lens combination performs, provided that you execute your tests in a technically solid manner (mirror lockup with remote release, perfect focus, etc). The Sharpest Image delves into some of these issues.
Those planning on a future of high-resolution DLSRs might well consider strategic investments in lenses like the Coastal Optics 60/4 APO macro, the Leica 90/180/280 APO series and/or some of the Zeiss ZF line, because those future high resolution sensors (24 - 36MP) will demand diffraction-limited performance by f/4 to realize the sensor potential.