According to the current issue (issue #22, Page 26 “Letters”) Full Circle Magazine at http://www.fullcirclemagazine.org, we can convert the existing ext3 to ext4 by the following command. However, I did not try it yet until the release of Ubuntu 9.04.
tune2fs -O extents,uninit_bg,dir_index /dev/DEV
ext4 is running fast and stable under Ubuntu 9.04 Alpha. I have installed it on a AData 300 Series 64GB Solid State Drive (SSD, MLC) with no problem. However, the BIOS should not select Advanced Host Controller Interface (AHCI). According to some source, ext4 is also good for SSD usage.