How to use Boot Camp Assistant to create a dual OS X, Ubuntu system? I would like to use Boot Camp Assistant to partition my HD, reserving around 30 GB for Ubuntu. I would then have a dual OS X Yosemite and Ubuntu Linux system.

This used to be fairly easy. Boot Camp Assistant would partition the HD, then you could quit, and then using some boot manager software like rEFIt (now rEFInd), you could use that reserved space to install Ubuntu. 

Now, I'm not so sure. Boot Camp Assistant gives me three options: 

(1) Create a Windows 7 or later version install disk
(2) Download the latest Windows support software from Apple
(3) Install Windows 7 or later version

For (1), I need a Windows ISO image. I cannot do with without a Windows product key.

(2) just downloads Windows support software to a 16 GB USB port of mine, and then...Boot Camp Assistant quits. 

(3) doesn't work, as apparently I need an installer disc.

How should Yosemite use Boot Camp Assistant to partition their HD? Why is Apple doing this to us?
Personally, I would avoid using BootCamp altogether.  I would look at VirtualBox as a free solution to run Ubuntu on your Mac.  If it does not provide everything you require, you might try a demo of VMWare Fusion for OS X.  It will support things like KVM Hypervisor in a VM, which VBox will not.

If you insist on a native Ubuntu running on bare metal, just use Disk Manager to resize your system partition, add a second partition and install Ubuntu on the second partition.  You will likely clobber your apple boot record, but you should be able to restore that by using the Recovery Boot Partition and repairing your OS X installation.

After you have Ubuntu installed on a partition, try to use the boot menu to select the partition from which you wish to boot.  You get the boot menu by pressing and holding the option key after the boot sound and before the grey screen.  It will present a list of system devices that are detected.  It will also include USB drives, if that is a path you wish to take instead.  Select the one from which you want to boot, and hit enter.

That should/might work.  I haven't used a dual-boot Mac in some time as I use VMWare instead.