Releasing a Core i7 iMac at the same time as the Mac Pro would have seriously affected sales at the top end.
I'll be pulling the trigger on the latest iMac, safe in the knowledge that by the time that has been retired, the 8 core iMac with OS X 10.8 would probably be out and application development for multi threaded applications will be nicely matured.
Between then an now, I expect a slow transitional period and buying hardware too soon for software to take advantage is premature in my opinion. Sure Snow Leopard will help things out in the near term, but developers will need time to adopt this technology. Adobe has yet to release Photoshop to the best of Leopards abilities!