Converting ext3 partitions to ext4 on Ubuntu 9.04 (Jaunty)

I've upgraded to Ubuntu 9.04 ("Jaunty Jackalope") from 8.10 ("Intrepid Ibex") about a week ago and was surprised how smooth it went. Jaunty's still in its alpha stage (Alpha 3 as of now) yet it's surprisingly stable. One thing that Jaunty, once officially released, will add to the regular Ubuntu is ext4 support. Ext4 is an upgrade from (wait for it.... you'll never guess!) ext3, which is starting to show its age in the filesystem world. Ext4 provides better performance than ext3, mostly due to its delayed allocation and more file support (like you'll ever have files bigger than 2 or 16 Terabytes?). Anyway, I felt like risking an in-place conversion of my two ext3 partitions for the sake of performance... and it went relatively smooth. I'll explain how I've done it so you can give it a whirl as well. One thing worth noting though is that once a filesystem is converted to ext4, there's no going back. You can't convert it in-place to ext3, you must format the whole thing and start over. So obviously, back your shit up.