ZFS was released with the CDDL license, so it cannot ever be built in to the Linux kernel.
It is possible to install ZFS on Ubuntu, and several people have released howto documents. If you Google search for "ZFS Ubuntu" you find several. Here's one: http://systembash.com/content/howto-installing-zfs-and-setting-up-a-raid-z-array-on-ubuntu/
ZFS on Ubuntu works with the "file system in userspace" (FUSE) system, rather than being a kernel module. I believe this means you will not be able to boot from ZFS, but you should be able to keep user data in it. Here is the hope page for the ZFS on FUSE project: http://www.wizy.org/wiki/ZFS_on_FUSE
ZFS has its own ways of being redundant, so yes it replaces md-raid and lvm.
Note that Btrfs is coming soon, and it will be the native Linux answer to ZFS. It is available now, but I would not advise you trusting important data to it. I expect that Btrfs will be solid in perhaps another year.
I will stick with ext4 on RAID until Btrfs is out of beta status. I would also be willing to install OpenSolaris on a spare computer and use it as a ZFS file server with NFS. I am reluctant to trust important data to beta software, and ZFS on FUSE is still considered beta.
EDIT: I'm updating this to recommend ext4 instead of ext3. Sadly, I think that BTRFS is still at least a year away from being ready, so I'm not editing that part. (BTRFS still doesn't have a working fsck that can fix errors yet!)