Ever since Google open sourced Ice Cream Sandwich, everyone is getting creative and trying to install it on variety of smartphones. A very important thing to note here is low hardware requirements of Android 4.0 i.e. Ice Cream Sandwich which allows you to make it run on older devices also like HTC G1. Yes, HTC G1 is very first phone to run Android Mobile Platform and have quite low hardware configuration.
Jcarrz1 from XDA-Developers has succeeded running Ice Cream Sandwich on HTC G1. Check out the video below to watch it working nicely. You can make calls and SMS. Apps are working fine. Only hitch is with Wi-Fi and Bluetooth. Of course this is an experimental attempt and a work in progress. So, if you are going to try it now, just keep these things in mind.
If you are thinking, whats the point running Android 4.0 on HTC G1, then you must understand that if G1 can run it, you most likely be able to run it on any other Android phone. So, if you own an android phone just wait a while and be ready for upgrade.