Thats right! Ext4 has been marked stable as of kernel 2.6.28. While we all know its only a stop gap until btrfs is out, it still offers some great performance benefits for little effort. More info on ext4 can be found here, along with a basic ext4 howto.
I decided i wanted to change the format of one of my Gentoo systems from reiserfs to ext4 and test this new fs out. If your going to do this make sure you have upgraded to kernel 2.6.28 and =>sys-fs/e2fsprogs-1.41.3
At the time of writing this, the only live cd with ext4 support i could find was fedora 10. However, the fedora live cd doesnt support reiserfs which is what my drive was already formatted in. So i had to boot from a Gentoo live cd, tar everything off to a server, then boot from the fedora cd, fomat in ext4 and then untar the data back.
Upon booting back into Gentoo, the newly formatted partition wouldnt mount. It insisted that i needed to enable CONFIG_LSF (needed for 2TB files) in the kernel because i was mounting a drive with large files... even though this drive was only 8gb in size! After i recompiled the kernel with CONFIG_LSF on it all worked perfect. I think this is something to do with e2fsprogs and i might lookinto it later.
Over all im very impressed with ext4 so far. While i dont have benchmarks to share, i defenately noticed a speed up on disk operations. Such as parrell booting in Gentoo and firing up KDE all seemed much quicker.
As a side note, it is also interesting that Ubuntu will be including EXT4 support as of 9.0.4 and is showing impressive boot performance... Info here