Got into a debate about the future of the Mac Pro on twitter. I believe this all started from an article Ted Landau wrote for Mac Observer
I haven't read the whole thing but that hasn't stopped me before. Here's what I think it would take for the iMac to be able to replace the Mac Pro. Do I think this is likely? Not really, I do think it's got an outside possibility of occuring. I'd give it about 30% chance of happening in 5 years. Also note that I do assume both lines will continue to be updated.
So, the Mac Pro has several advantages over the iMac. Most of them would have to be overcome for the iMac line to bypass the Mac Pro.
Currently the Mac Pro maxes out at 4 core Xeon (the Xeon means it's capable of hyper threading where each core can act as 2 cores, effectively making the current Mac Pro an 8 core system). The next update will probably double this.
iMac matches the current Mac Pro. Top end iMac is a 4-core i7 which has hyper threading. Will the next iMac update be able to match the Mac Pro's next update again? My guess is yes, in a year. but the real question is: does continually throwing cores at your software scale linearly or is there a point where no additional processors will improve your speed. My guess is there is a maximum but it's probably more limited by drive speed than anything else. 128 core processes seeking to store their work on the same drives simultaneously will probably choke waiting for drives, while 2 64 core processor machines (with separate drive systems) complete the same task faster.
This is going to come down to the nature of the work. Fortunately for the Mac Pro, it's primary use in video and science research are easily parallelized on to multiple processors. It may be a lot of iterations of processor upgrades before the a peak is hit on these systems.
Maximum RAM In 2006 the differential between max memory in the iMac line and the Mac Pro line was 16x (iMac - 2GB, Mac Pro - 32 GB). Today it stands at 2x, and after the next Mac Pro update will probably be 4x. It appears the iMac is closing in on the Mac Pro. If the next update jumps to a 128GB max instead of 64GB, then it may be able to keep it's advantage a lot longer.
Maximum video displays iMac is at 2, Mac Pro is at 8. Will future upgrades add more to the iMac, will users demand more than 8 on a single computer? This is will be doable in the near future and mostly comes down to: Apple will do it if they really want to replace the Mac Pro line with an iMac Pro line instead (and if Apple wants to do this is completely up to Apple, who hasn't asked me.)
Maximum drive space The only way the iMac can compete here is if there is some new external bus speed, faster than eSATA and Firewire 800, that enable the iMac to access an expansion system faster than an internal system. Rumors abound Apple will be going to the Light Peak interface, replacing all Firewire and USB connections, but Apple rumors gather like flies and are about as reliable. I think a new external bus sufficient to overcome current internal drive speeds is likely, but 1 year, 2 years, 10 years?
Also this assumes there won't be some new internal bus system that is far superior than the internal. I actually think this will be true. SATA/eSATA point to systems going to the same bus technology internal/external and I think that will continue.
Specialty expansion cards Want fiber channel cards or speciality rendering cards? Mac Pro is your only hope. Only 2 ways an iMac could over come this: some brand new, super fast, not even rumored, external card system comes along (express card is insufficient), or the iMac gains so many processors that dedicating them to some of these speciality tasks is a no brainer.
We'll probably see some advances to express card, but not enough to overcome internal slot advances. Only hope here is for the the need of speciality cards to be reduced via additional processors, more memory and
When you have a big open volume of space and lots of fans you can run things hotter. Which means they can either run faster or you can run more of them. Rejiggering the case a bit to make it bigger (something Apple hates doing, they much prefer going smaller) might help. That might lead to a product line like the laptops, where MacBooks are the small ones and MacBook Pro are the big. iMac and iMac Pro, with the iMac Pro form factor being slightly larger.
Better power dissipation and higher efficiencies help as well, but that just lets you pack more in the larger case too. Moving to an internal SSD hard drive,with the large capacity drives being stored external would be a benefit. Assuming the SSD drive capacities expand as expected (pretty likely.)
So after all this, the summary -- the only real hope for the iMac to overcome the Mac Pro is if there is a point where the hardware is so ahead of the software that throwing more cores at it won't help, and new technologies come out that the advantage of external vs. internal is significantly reduced.
Likely? As I said, not really. Possible? Yeah, a faint possibility.