^^^^IMHO, really doesn't matter whether the nMP has an Nvidia option or not.
1. Nvidia writes driver and Cuda Updates after each Apple OS Update.
Those drivers are written associated with specific products. For example
http://www.nvidia.com/object/macosx-313.01.02f01-driver.html
on the supported products tab is:
GTX 680 for Mac
GTX 285 for Mac
GT 120
8800 GT
K5000 for Mac, 4000 for Mac
FX 5600, FX 4800 for Mac
Those are all products that went to market for sale. For example, updates in 10.8.3 arrived around the same time as
https://www.macrumors.com/2013/04/0...tx-680-mac-edition-graphics-card-for-mac-pro/
Coincidence?
If there is no card vendor asking them and Apple isn't asking them then there is little to no demand allocate development resources this to cover a broad spectrum of new cards and implementations.
2. Both the MacBook Pro and the iMac have models that use Nvidia Graphics.
Nvidia has multiple implementation per major architecture rollout. A GK110 is not a GK104
The normal driver distribution path for embedded Mac graphics is through Apple.
So yes there will be a baseline to work with for the corresponding top end iMac GPU (likely an deeply underclocked desktop model ). However, the support is highly limited and variable.
The next iteration of the MBP/iMac will have another design bake off for embedded GPU. If Nvidia manages to loose both that and the next Mac Pro round of its design bake off .... then new cutting edge card drivers for public consumption aren't going to be a high priority.
Does Apple want Nvidia to loose the design bake offs? Probably no more than they want AMD to loose. Multiple vendors vying for design slot wins helps Apple deliver better products over time. But if Nvidia doesn't bring what Apple wants and at the price they want then Nvidia won't get the win.
[ In the two year iteration before 2012-2013 AMD had wins in MBP/iMac space so it is possible to wrap up all of Mac line up if have right designs at the right time at the right price. ]
3. Apple really doesn't (and hasn't) offered for a while now, reasonably priced Nvidia graphics cards for the current Mac Pro.
Goes back to #1. If limited demand has killed off 3rd card vendor interest then where's Nvidia's motivation? What has typically happened is one vendor takes a stab (recently EVGA) and then firmware and drivers go rogue. This results in the numbers sold and margin is so small (have to use discounts to move the cards) the Apple store doesn't want to bother to stock them.
if there was a demand problem of cards priced "too high" before moving them to a external PCIe enclosure isn't going to make them any more cost effective.