Storage isn't the only thing that matters. 24" iMacs were limited to laptop CPUs (well, custom TDP but laptop performance) which made a huge difference between iMac and Mac Pro. Back then, people had to go for Mac Pro to get a decent amount of CPU power and RAM.
My comment was for those that examined their usage, and determined that the fixed CPU, GPU, max RAM capacity, and monitor screen were sufficient for their needs. In such cases, the final deciding factor could be storage (speed, capacity, or both).
27" iMac has changed this. Desktop CPUs and four RAM slots, and the iMac is pretty darn close to the base Mac Pro.
Those that could live with an iMac to begin with, absolutely.
But when we're talking about pros trying to switch from an MP to an iMac, I'm not sure the cannibalization is all that much up to this point (let's say a couple of thousand units), due to the lack of Thunderbolt peripherals actually available. Only those that either didn't need the MP to begin with, or were right on the edge when they bought their last MP for example.
I think of it this way; for those that the current iMac won't quite make it for their needs as-is, but would with TB based solutions to make up for the short comings (better GPU, storage, ..., to importing TB camera data).
Given there aren't many TB peripherals in the marketplace yet, those that could be in this position will wait to switch over to the iMac after these peripherals actually show up (storage products, such as the Promise Pegasus boxes are out, but no GPU solutions or even multi-interface Hubs yet).
Now if the peripherals do show up as promised (and work properly), and aren't hideously expensive, then the TB equipped iMacs will cut deeper into the SP MP sales figures (including the next SP MP when they release). Combining this with further improvements in CPU and GPU performance, it could displace the MP entirely (say once consumer desktop CPU's hit 8 cores per die).
Hopefully this clears things up a bit.
Not to say that iMac has taken all Mac Pro sales, but I would still claim that the 2009 numbers are bigger than 2010 and 2011. Storage is starting to be the only advantage of Mac Pro (along with PCIe cards), and even that is now questionable due to Thunderbolt.
I'm not saying that the 2010 numbers are larger than 2009, as we don't know.
What I am saying however, is I don't think the cannibalization is all that large at the moment. Say a couple of thousand units for those that wouldn't have been able to use an iMac before, not those that overbought with an MP to begin with = shifted to the iMac due to the cost increases on the 2009 or later SP MP's (think enthusiasts who usually would have bought the MP, yet never stuffed a single peripheral in it, were priced out of the LGA1366 based MP's - and they do mostly web browsing for example).
