The HTC G1 is the original Android smartphone. It wasn't exactly an amazing start for Android, but it got quite a few early adopters because they believe in the idea of an open source mobile OS to unify the manufacturers, by getting all of them to use it. Android has succeeded, and therefore the G1 remains an important part of Android's history.
To show what they are capable of, custom ROM developers have ported the Android 4.0 ASOP to G1, which is a pretty amazing feat, considering that it's a 3 years old phone with very old hardware. It had a 528 Mhz ARM11 CPU and a Adreno 130 GPU. Yes, you read that right. There was actually a weaker GPU than Adreno 200 before it, and it was in the G1.
The ROM doesn't look like it's terribly smooth so far, definitely not like it is on the Galaxy Nexus, but what can you expect when it has to be hardware accelerated by that ancient Adreno 130 GPU? But hopefully, this is mostly due to being an initial alpha version, and that it can work a lot better later on.
The reason I'm saying that is not necessarily because I want Android 4.0 to work extremely well on the G1 itself, and I don't think too many people own it anymore, but because if it does work very well on that chip, then there's hope for current low-end Android phones, too, which come with a 600-800 Mhz ARM11 CPU and Adreno 200 GPU.
It remains to be seen if we'll see Android 4.0 on that ARMv6 architecture at all going into 2012. My guess is that no low-end phone will receive an update to it, not just because of the old chip, but also because of the very small internal storage, which probably makes it impossible to install Android 4.0 on it without cutting a few apps from it, which I'm guessing it's what these ROM developers have done for the G1 version, too.
But also because the ARMv6 architecture on which the ARM11 chip is made is so very old, and I think going into 2012 we'll start seeing at least the pretty unknown Cortex A5 chip, that's supposed to be somewhere between ARM11 and Cortex A8 performance wise, but with much better battery efficiency and is also much cheaper. Plus, Cortex A5 is built on ARMv7, just like Cortex A8, A9, and the upcoming A7 and A15.
Another option would be waiting it out until cortex A7 starts arriving in low-end models sometime in 2013, and not putting Android 4.0/5.0 on any low-end phone until that happens. They would continue to get only Android 2.3. That seems like a whole lot of time to wait for it, though, so maybe manufacturers will do their best to make it work properly on the low-end phones, too.
I'm sure a lot of people who are more price sensitive in general and would get those types of Android phones, would be much happier with the much more polished and intuitive Android 4.0 version, too.