ext4 vs yaffs seems to be a spurious debate in this context: YAFFS2 is suited to raw nand storage devices which have very different requirements to conventional hard disk-like media (such as SD cards) which require no wear levelling, ecc and external accounting for bad-blocks.
Many current generation android devices are configured with a smallish raw nand device at the root of the filesystem + a larger micro SD card (e.g. Motorola Droid) for user multimedia data etc. The trend seems to be going towards fitting phones with a large SD card-like flash chip (Google nexus S), which requires no external wear levelling and thus is suited to conventional filesystems like ext2,3,4 etc.
The quality and reliability of the firmware in these SD card-like flash chips (MoviNAND, iNAND etc) varies of course.