That is interesting since the processor in the N1 is not much slower if at all from the Nexus S but that S will get ICS OTA.
From some other reading I have done it appears to be memory related. In a stock N1 it's pretty easy to run out of memory even if you move as many apps to the SD card as you can. The only way to alleviate that issue is to install a custom ROM and then you can move just about anything.
For me I guess I will decided how important ICS is for the phone (I am sure it will be on the Xoom tablet I have) and make a decision accordingly.