You'll notice with roms like CM and the AOKP that they rarely ever work out of the box. Generally code tweaks have to be made and those code tweaks are allowed to be distributed because they are owned by the project, not by a vendor.
The point I was trying to make was that if google wanted to, they could release AOSP for the CDMA phones even if they used older modem images. For that matter, they could do exactly what CM/AOKP/etc all do, and just not include the modem images at all. In a way, that might actually pressure the carriers to move quicker on approving modem updates, etc - the customers would see firmware (it's not read-only) updates from google, and wonder why their carrier isn't updating the modem.
It can work and has worked for the last 5 months. My verizon GN is running 4.0.4 with 4.0.2 modems. Nothing special had to be done.
The only reason I've seen from google on why they won't do this is that certain packages that need to be included in the firmware must have certain digital signatures - so users can't build those packages themselves. Google hasn't given a reason why they can't distribute those same packages in binary form (actually, they do already but not officially.)
To me, that sounds like Google is saying: We can't sign it ourselves, and you can't sign it, therefore we won't support it. If google isn't supporting it, then it is not a "google nexus" phone. It might still be called a "nexus", but it's a VERIZON nexus (or a Sprint Nexus, or perhaps a "qualcomm nexus."
All that being said, I'm not really complaining. I purchased my "verizon nexus prime" just a few weeks ago and I don't regret it. I might have purchased the real GSM "google nexus" if it was available with 32GB of RAM (sorry, but 16GB and no memory slot is just too limiting.) (I wonder if the lack of a 32GB GSM nexus is the result of a contract...) Regardless, I'm running 4.0.4, and have been for weeks. Google might not be supporting me, but they probably wouldn't have supported half the stuff I do with my phone anyway.
What is a Nexus? Just a name and nothing more. I don't think the verizon phone is a real "google nexus" (for the reasons above), but it's pretty close.
Edit: correction, I'm running the leaked FA02 modems.. which apparently are independent of any specific version of android.
Gary