It'll work as a Live CD and installed on a separate partition. I believe there is a small work around to get the partition editor to work right so you can install it onto a Mac.
You will, but you won't be able to use Ubuntu unless you have a keyboard with wires on it (or of course the internal keyboard on a notebook computer). It doesn't affect very many people. But if your setup is like my iMac, which has just a Apple Wireless KB and Mouse, then you can't use Ubuntu on it.