Even a spec bump at this point needs more fanfare than a quiet update. The so called 'update' to the MacPro that occurred with the MacBook Pro refresh wasn't really an update at all. Apple was forced to replace the CPU's in the aging Mac Pro line due to the fact that the processor was so old, and had been out of production for so long, that THERE WEREN'T ANY LEFT.
If we see a silent refresh to the iMac line, it will probably be something similiar to this, with incredibly modest tweaks to maintain production in light of diminished supply of aging tech.
No silent refresh. Fanfare! Huzzah! Ivy Bridge! 7970M/680M GPU! USB3!