From the comments responding to the announcement, here is how it can be made to install on custom ROMs. Just tested this:

Just run the following commands to uninstall the old system apps if you use a custom rom.

adb remount

adb shell rm /system/app/GoogleQuickSearchBox.apk
adb uninstall com.google.android.googlequicksearchbox

adb shell rm /system/app/VoiceSearch.apk
adb uninstall com.google.android.voicesearch

This will let the new ones install from market without the package signing error