GRUB is your bootloader -- the first piece of software to start after you switch on your computer, and responsible for loading the operating system.
Grub 2.00 has not been tested by Ubuntu and any problems may render your system unbootable--including Windows if dual-boot--and will require a LiveCD/LiveUSB and time to fix.
Unless you simply want to experiment, it is highly recommended to wait for Grub 2.00 to officially make its way into Ubuntu (or at least until 2.00 shows up in Debian Sid or the Quantal alphas!)
That said, Linux/FOSS is about freedom, and if you get the gun, I'll teach you how to shoot! :)
If you still want to install Grub 2.00 after the warnings above:
You'll have to build it from source. Open the terminal and type/paste:
sudo apt-get build-dep grub2
wget -O- ftp://ftp.gnu.org/gnu/grub/grub-2.00.tar.xz | tar -xJ
sudo make install
Now, you need to install the new Grub to where your existing Grub is. If you don't know where or how to find out, quit now!
Reboot, and...it's slightly different...
Advanced Options gives us the reassuring....
But "GNU/Linux" seems to work anyway...
No guarantees, but
sudo apt-get --reinstall install grub2 should overwrite Grub 2.00 with the official 1.99x version.