Android File Systems

YAFFS
Android in most mobile phones up to version 2.2 (Froyo) use the YAFFS2 filesystem.

Here is some information about YAFFS2:
 * File Systems
 * Wookey's presentation about YAFFS at ELC Europe 2007 [[Media:yaffs.pdf|yaffs.pdf]], video
 * YAFFS update for ELC Europe 2010

EXT4
According to this report by Ted Tso, version 2.3 (Gingerbread) will be using ext4

http://www.linuxfoundation.org/news-media/blogs/browse/2010/12/android-will-be-using-ext4-starting-gingerbread

With ext4, you may need to explicitly sync the data to the file system, in order to make sure not to lose information.

See this blog entry by Tim Bray for information on this topic.