I think it is likely that as soon as the mobile Penryns become available, Apple will begin shipping both MacBook Pros and iMacs with them.
The Penryn desktop CPUs are expected before the Penryn mobiles, and so Apple will be marketing Merom based iMacs against Dells etc which are will have desktop Penryns.
So if the mobile Penryns are really going to be shipping in Jan 08, then I think we can expect an iMac and MacBook Pro upgrade announced at MWSF 08.
However, I'm a little dubious about the January date for the mobiles. The Penryn Xeons don't ship until mid November 07, followed by the desktops and then the mobiles. It was only a week or two ago that the mobile Penryns were tipped to be released around April 08.
I've waited long enough. My C2E iMac will be ordered the day that Leopard is bundled with them. I'll then wait until either the MacBook or MacBook Pro are on Penryn and get a laptop.
So you think it's likely that the recently updated iMacs will be updated again in January with Penryn processors? I too was planning on ordering a Core 2 Extreme iMac once Leopard is bundled with it. I wonder how much faster and energy savings will be realised in a Penryn iMac versus a Core 2 Extreme iMac. Anyone have ideas?