I am interested in installing Whatsapp on Android 1.6 (Cyanogenmod 4.2) on a rooted HTC Dream G1. Is there any way to install it?
From the whatsapp site, the minimum requirements are Android 2.1, so even if you managed to find the APK and side-load it onto the G1 running 1.6, it probably will not work.
Stable releases of CyanogenMod 5 (Android 2.1) and 6 (Android 2.2) are both available for the G1. You can install Whatsapp if you upgrade to CM 5/6.