When looking at the slides that have been released regarding Haswell, or the info floating around from IDF/Hotchips, Haswell is seeming less impressive than Broadwell. Haswell adds FMA3 and some other instructions, alongside having a better graphics processor (though it's unclear just how much better, GT3 notwithstanding). On the other hand, integer/float performance is at best going to be incremental. Broadwell, though not a new architecture per say, is where substantial changes are going to be taking place between the chipset, performance (and especially graphics) improvements, and a large drop in power consumption. It's fairly common to see rather hopeful commentary farther out, and then more realistic expectations near the release, but Broadwell on paper appears to be the processor to look out for, not Haswell. So, I don't think waiting on the 2013 iMac is really going to net one anything except a bit more speed and minor incremental updates (if any).