Use of EXT4 will begin with Google's Nexus S smartphone.
Google's Developer Advocate, Tim Bray, and Kernel Developer Ted Ts'o both confirm in theirblog posts that the move from YAFFS to the EXT4 filesystem is underway and Google's Nexus S with Android 2.3 will be one of the first smartphones to use the EXT4 filesystem.
More information on Android 2.3 and the EXT4 filesystem can be found on the Android Developers website.