Valerie Aurora over at Red Hat has just posted on her blog about ext3 and ext4 and fsync() issues we've all heard so much about. As she says there, rename in ext4 now implies fsync() so that issue should calm down.
However, 2.6.30 is defaulting to data=writeback, which means it only writes the metadata to the journal—not the actual data. This is how XFS, ReiserFS, and a few others work, and it's much faster than ext3's default data=ordered. It's also somewhat less awesome at ensuring your data doesn't get lost. She's asking that people test patches (linked from her blog) for a new journal mode called "guarded" (created by Chris Mason) which she says will be faster than "ordered" but still have its data consistency guarantees.