Intel is presenting is new Nehalem-EX CPU with 8-core and 16-threads.
It’s really likely that we will see it on refreshed Mac Pro line up, with 8-core 16-thread mono-cpu basic system, and 2-cpu 16-core 32threads system on custom design (as well as 6-core 12-threads or 4-core 8-thread entry-level customized Mac Pro).
Hope to see it fast 
I wrote that the new Core i5 27″ iMac offers same *GENERAL* level of performance than *STANDARD* Mac Pro configuration. It means that out-of-the-box, both will provide same average performance level on a bench suite, with an advantage for the iMac. And I disagree with Gizmo’s conclusions about Mac Pro.
The fact is your work with your Mac is *NOT* the average benchmark, and you may need faster graphics, faster CPU or bigger RAM to achieve your task. There you may go for the iMac for it’s quality or choose the MacPro that is more balanced and more expandable.
Where the Mac Pro shines, even with same quad-core/8-thread processor as 27″ iMac, Core i7/Nehalem, is where you need as much memory as possible (big Photoshop work for printing, many Virtual Machines, Big Lightroom database, …) or where you need faster hard-drive or SSD, because in any of our actual Mac the hard-drives are just a bottleneck. Naturally you may want to use 3D software or OpenCL software and install 2 or 3 GPU/Graphic card.
Buying an entry-level Mac Pro, that is not faster than an iMac is not a bad choice, if you could afford it and plan to expand it to your needs.
For myself, I plan to buy one, entry-level system or even 4×2.93Ghz, with the idea to upgrade memory at 12GB for big development (iMac could cope!), multiple video card, Radeon 4870 (gaming and OpenCL) and GeForce GT120 and GT130 (CUDA and OpenCL), and from start installing 4×1TB hard-drive with software RAID-0 to be able to exploit the incredible level of performance of the CPU without being limited by hard-drive slowliness!
No iMac will enable me to develop on different graphic card at once, for CUDA and OpenCL, no iMac will enable me to have 400MB/s bandwidth from hard-drive (will be around 100MB/s instead), no iMac will enable me to have external hard-drive connected in eSATA or in USB3 in the future, so Mac Pro is the way to go for me!
As I expected, even with Core i5, the new 27″ iMac is faster than quad-core Mac Pro, and even 8-core Mac Pro, as benched by MacWorld. Moreover, the Core i7 version adds a mere 8% to the overall performance!
If you don’t need fast hard-drive in RAID, ability to put more than 16GB RAM (that is plenty of RAM even by today’s standards) or ability to have fastest graphic card (OpenCL oriented, like GeForce 8800, GT120 or GT130), the new 27″ iMac Core i5 is a fantastic performer with the right price tag!
I knew for sure the iMac Core i5 will beat the quad-core Mac Pro, and I am not deceived this time