2.3 Gingerbread will use ext4 instead of YAFFS.
My question is: YAFFS is for flash based filesystem, but ext4 is not. Will it lead to premature death of the device due to wear leveling?
Not likely, the flash storage itself usually have built-in hardware wear leveling, YAFFS2 is only advantageous when the flash storage does not have hardware wear levelling.
However if you're reformatting a flash storage, do check that the flash storage have hardware wear leveling; most flash storage that originally ships with ext4 (e.g. Nexus S) should come with hardware wear leveling, but if the device originally ships with YAFFS2 or other file systems that does not do wear leveling, they might not have hardware wear leveling. On these file systems, you might need additional steps to ensure that a lower level driver do software wear leveling if you want to safely install ext4 (or other filesystems that does not do wear leveling).
Where did you see that all Gingerbread devices will use ext4?
The Nexus S uses ext4 and that is because the Galaxy S devices uses a very slow and laggy file system developed by Samsung called RFS. Google decided to change the file system on the Nexus S from RFS to ext4.