As noted above, a number of these patches force changes to the on-disk data structure. According to Ted, though, these should be the last on-disk changes for ext4. There are some features which still will not have been merged when 2.6.25 comes around - delayed allocation and online defragmentation among them - but they should not require format changes. So ext4 is getting closer to the point where it is considered ready for production use.
It is not at that point yet, though, and people who use it are still doing so at their own risk. To help drive that point home, Ted has proposed a new mount flag (called test_fs) which communicates to the kernel the user's understanding that they are about to mount a developmental filesystem and will not go filing lawsuits if things go wrong. In the absence of this mount option, an ext4 filesystem will refuse to mount. One might think that child-proofing the filesystem in this way would not be necessary, but some extra care in this area can only be a good thing. Filesystem-related surprises are rarely welcome.