I think it's mostly the latter. There's no point in bumping the pixel density of the display, and the weight and thickness are fast approaching the point of diminishing returns. Apple's even been relegated to lying about the thinness. They're calling the iPhone 5 the "thinnest smartphone in the world", but it's clearly not the case.
Spec-wise, everything in the iPhone 5 could have been achieved when the iPhone 4S came out. It's becoming apparent they're holding features back just so they have an excuse to release the next version. You can bet the iPhone 6 is going to have NFC and wireless charging, even though they downplayed the usefulness of those features today.