It's not simply a spec-bump. Apple could have done it ages ago. Dropping a new CPU with the same thermal envelope, a new GPU (may be a few watts hotter), and a SSD standard (on the high end model at least). That sounds like a really hard problem!
In this age and time of obsession with all things Apple, exterior design matter and Apple knows that full well. Look at the iPhone, all the leaks have been cases, back panels, front panels, display, etc. All external components. Why? Cause people really want to know what Ive has come up with. I don't recall ever seeing a picture of the A6 CPU before the announcement.
The iMac G5 first made the scene in 2004. The same plastic design went unchanged until 2007, when the aluminum iMacs came out. It has been 5 years since and I bet you Apple is looking for a refresh of the line. At the very least, we know something is different about the display with all those "lamination issues". So it's not just a spec bump.
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They are not sure about the release dates for some iMac models. Pretty consistent with previous rumors. Only 16 hours to go before we know for sure. If Apple doesn't release the new iMac line tomorrow (regardless of stock availability), don't bother releasing it this year.