And the replies:
Coming from someone who spends all his working hours porting games to the Mac hope I can put this to bed,

the lack of an upgradable graphics card in the New MacPro has no effect on decisions on porting graphically intensive titles.
As 98% of the current Mac range cannot be upgraded graphically Apple's move with the Pro to make this 100% has very little effect on the overall status of Mac GFX cards. Given the cards in the pro are extremely powerful and will last for years in terms of gaming support it's highly unlikely the Pro will be under spec due to graphics cards, when that happens the Pro will have to be very old.
The Pro will ship with 2 AMD FirePro cards which from what specs have been leaked means:
2 x 2048 Stream Processors
2 x 384 bit Memory Bandwidth
2 x 6 GB VRAM (depending on spec)
~7 TFLOPS (OpenCL) processing power
To put this into perspective the 680GTX high performance PC Gaming card generates 3TFLOPS and the AMD 7970 PC gaming card 3.7TFLOPS. The FirePro cards are basically similar to the 7960 in many ways and the Pro has two of them! So it will start off with twice the graphics power as the super high end PC gaming cards.
For reference the HD5000 scores 0.7 TFLOPS and the NV 650M in the retina also scores a very similar 0.69 TFLOPS. This means the MacPro has roughly 10 times more graphics power than the latest rMBP and MBA machines!
I know TFLOPS is not the be all and and all of performance but it is a good indicator of the potential of the device. Actual performance can be effected by other things like VRAM speed and drivers etc
Edwin