In the past it was not possible to install Ubuntu on a pure Btrfs file system (/ btrfs + swap), will it be possible for 12.04?
You have been able to install Ubuntu into btrfs since 10.10
See the following guides
With that said, see the stability section on the wiki
You can boot a btrfs root (/boot) partition with grub2 (1.99), but the initrd has to be compressed with zlib (not lzo)
Looking as /etc/initramfs-tools/initramfs.conf, gzip appears to be the default, you should be good to go.
And from the ubuntu wiki page I gave you :
So install Ubuntu 11.04 or higher, be sure to install grub to the MBR (this is the default behavior of the installer).