The 5750 is not suitable for a base card in the manner Apple have historically done things. Too much "waste" in cost, power, temperature, unused GPU power when a good portion don't need any 3D capabilities.
If they continue how they have the last four times then it'll probably be a Radeon 5600 series as the base card and a GeForce 400 series. The other possibility is they don't switch them round this time and maybe we get a GeForce 220 or 230 and the Radeon 5870.
At idle the Radeon HD 5750 is one of the least consuming graphic cards out there, as well as under load. The price also seems to be inline with what other cards have been priced at for the Mac Pro.
Geforce GT 7300 ($100 to $140).
Radeon HD 2600XT ($90 to $150).
Geforce GT 120 (OEM-only product, so not sure about the price, but it is just a rebranded Geforce 9500 GT $89 to 99).
Radeon HD 5750 ($109 for the 512MB version).
This seems to fall squarely in the same price bracket as other products used in the Mac Pro.
The great thing about including Juniper (Radeon HD 5700 series) support for Mac OS X, is the fact that Juniper is simply half of what Cypress (Radeon HD 5800 series) is.
It would also make sense with regards to OpenCL to have a 720 Steam Processor part as the base card in a workstation.
You seem to be looking at it from a buyer's perspective, not a business one. Apple will likely continue to supply a card that costs them the least while offering "enough" performance. The 5750 is a good card in the higher end of the midrange, but Apple haven't used something from that section previously. All the cards that scored better in the temperature and power tests on that link are more likely candidates sadly.
I suspect that the low end card will continue to be a Nvidia and the BTO will be the 5870. I just don't see them changing that at this point, given what's being offered.
Historically, wants you to have an option from both vendors.