HTTP2 when it comes (2014?) will negate the need to inline/concatenate CSS or cook up image sprites because it solves it at the proper level, the network layer.
If you load large amounts of render blocking JS/CSS in the <head>, the browser must wait for it to finish downloading before it can render content in <body> to the screen.
To deliver a fast experience, keep the amount of JS/CSS needed to render the initial view to a minimum. Ideally, that'd be no JS, and just the CSS needed to style the content in the initial view. Then, once the initial view has rendered, load the JS and additional CSS needed for the rest of the app.
Unfortunately the question then becomes, how long until all the major browsers support it? And once that happens, how long until the crowd not using the updated browsers is small enough that we can safely ignore them?
I suppose the auto-updating browser trend should help a lot with the latter issue.
So (pardon my ignorance, I googled a bit but didn't find very conclusive results), why isn't everyone using HTTP2/SPDY now? Because of IE < 11 and Safari?
Edit: Or maybe because of the SSL requirement? Edited again: For clarity.